ges: fix G_DECLARE_FINAL_TYPE -Werror with clang

Also fix wrong casing the `G_DECLARE` for GESDemux.

../subprojects/gst-editing-services/plugins/ges/gessrc.c:56:1: warning: unused function 'GES_SRC' [-Wunused-function]
G_DECLARE_FINAL_TYPE (GESSrc, ges_src, GES, SRC, GESBaseBin);
^
/usr/include/glib-2.0/gobject/gtype.h:1405:33: note: expanded from macro 'G_DECLARE_FINAL_TYPE'
static inline ModuleObjName * MODULE##_##OBJ_NAME (gpointer ptr) {                                     \
                              ^
<scratch space>:39:1: note: expanded from here
GES_SRC
^
../subprojects/gst-editing-services/plugins/ges/gessrc.c:56:1: warning: unused function 'GES_IS_SRC' [-Wunused-function]
/usr/include/glib-2.0/gobject/gtype.h:1407:26: note: expanded from macro 'G_DECLARE_FINAL_TYPE'
static inline gboolean MODULE##_IS_##OBJ_NAME (gpointer ptr) {                                         \
                        ^
<scratch space>:42:1: note: expanded from here
GES_IS_SRC
^

../subprojects/gst-editing-services/plugins/ges/gesdemux.c:50:1: warning: unused function 'GES_Demux' [-Wunused-function]
G_DECLARE_FINAL_TYPE (GESDemux, ges_demux, GES, Demux, GESBaseBin);
^
/usr/include/glib-2.0/gobject/gtype.h:1405:33: note: expanded from macro 'G_DECLARE_FINAL_TYPE'
static inline ModuleObjName * MODULE##_##OBJ_NAME (gpointer ptr) {                                     \
                              ^
<scratch space>:72:1: note: expanded from here
GES_Demux
^
../subprojects/gst-editing-services/plugins/ges/gesdemux.c:50:1: warning: unused function 'GES_IS_Demux' [-Wunused-function]
/usr/include/glib-2.0/gobject/gtype.h:1407:26: note: expanded from macro 'G_DECLARE_FINAL_TYPE'
static inline gboolean MODULE##_IS_##OBJ_NAME (gpointer ptr) {                                         \
                        ^
<scratch space>:75:1: note: expanded from here
GES_IS_Demux
^
19 jobs for master in 71 minutes and 54 seconds (queued for 22 seconds)
Status Job ID Name Coverage
  Preparation
passed #540979
gst indent

00:00:20

passed #540978
manifest

00:00:36

 
  Build
passed #540988
build cerbero android universal

00:12:45

passed #540989
build cerbero cross win32

00:08:06

passed #540990
build cerbero cross win64

00:07:45

passed #540987
build cerbero fedora x86_64

00:06:34

manual #540992
gst-ios-12.2 allowed to fail manual
build cerbero ios universal
passed #540991
gst-macos-10.14
build cerbero macos x86_64

00:16:48

passed #540980
build fedora x86_64

00:07:22

passed #540986
docker windows 1607
build msys2

00:13:55

passed #540981
build nodebug fedora x86_64

00:05:39

passed #540982
build static fedora x86_64

00:09:36

passed #540983
build static nodebug fedora x86_64

00:11:45

passed #540984
docker windows 1607
build vs2017 amd64

00:11:55

passed #540985
docker windows 1607
build vs2017 x86

00:10:44

 
  Test
passed #540993
check fedora

00:04:14

passed #540994
integration testsuites fedora

00:13:16

 
  Integrate
passed #540995
android universal examples

00:07:15

passed #540996
documentation

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