srt: Fix GCC function type error

gstsrtobject.c: In function ‘gst_srt_object_close’:
gstsrtobject.c:1036:7: error: function called through a non-compatible type [-Werror]
       (GDestroyNotify) g_closure_unref);
/usr/include/glib-2.0/glib/gmem.h:121:8: note: in definition of macro ‘g_clear_pointer’
       (destroy) (_ptr);                                                        \
        ^~~~~~~
gstsrtobject.c:1038:7: error: function called through a non-compatible type [-Werror]
       (GDestroyNotify) g_closure_unref);
/usr/include/glib-2.0/glib/gmem.h:121:8: note: in definition of macro ‘g_clear_pointer’
       (destroy) (_ptr);                                                        \
        ^~~~~~~

Arch Linux
gcc 8.2.1 20181127
glib 2.58.2
8 jobs for srt-functype in 61 minutes and 30 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Preparation
passed #80170
gst indent

00:00:42

passed #80169
manifest

00:00:37

 
  Build
passed #80171
build fedora x86_64

00:03:33

 
  Test
passed #80172
check fedora

00:07:16

 
  Full Builds
passed #80173
build android arm64 api28

00:02:29

passed #80175
build cerbero cross win32

00:40:08

passed #80176
build cerbero cross win64

00:06:37

passed #80174
build cerbero fedora x86_64

00:43:19