meson: disable warnings for unused static inline functions with clang

These prevent us from using G_DECLARE_*_TYPE macros outside of
header files, as these generate static inline functions for boilerplate
macros (eg GST_IS_<OBJECT>).

https://gitlab.gnome.org/GNOME/glib/merge_requests/1062 has been
proposed in GLib, but we can use this as a workaround for now.

Taken from https://lore.kernel.org/patchwork/patch/796288/
30 jobs for clang-static-inline in 55 minutes and 6 seconds (queued for 35 seconds)
latest
Status Job ID Name Coverage
  Preparation
passed #585919
gst indent

00:00:37

passed #585918
manifest

00:01:05

 
  Build
passed #585929
build cerbero android universal

00:15:23

passed #585930
build cerbero cross win32

00:10:07

passed #585931
build cerbero cross win64

00:06:48

passed #585928
build cerbero fedora x86_64

00:06:57

manual #585933
gst-ios-12.2 allowed to fail manual
build cerbero ios universal
passed #585932
gst-macos-10.14
build cerbero macos x86_64

00:08:44

passed #585924
build clang fedora x86_64

00:07:29

passed #585920
build fedora x86_64

00:07:01

passed #585944
docker windows 1607
build msys2

00:11:00

passed #585921
build nodebug fedora x86_64

00:04:16

passed #585922
build static fedora x86_64

00:12:12

passed #585923
build static nodebug fedora x86_64

00:03:47

passed #585925
docker windows 1607
build vs2017 amd64

00:09:42

passed #585926
docker windows 1607
build vs2017 x86

00:09:02

failed #585927
docker windows 1607
build msys2

00:00:03

 
  Test
passed #585934
check fedora

00:04:39

passed #585935
integration testsuites fedora

00:19:34

passed #585939
valgrind bad

00:14:18

passed #585936
valgrind base

00:07:37

passed #586186
valgrind ges

00:08:46

passed #585937
valgrind good

00:13:05

passed #585938
valgrind ugly

00:02:35

failed #585940
valgrind ges

00:07:44

 
  Integrate
passed #585941
android universal examples

00:08:52

passed #585942
documentation

00:05:31

failed #586686
gst-ios-12.2 allowed to fail manual
ios universal examples

00:00:06

failed #586685
gst-ios-12.2 allowed to fail manual
ios universal examples

00:00:06

canceled #585943
gst-ios-12.2 allowed to fail manual
ios universal examples

 
Name Stage Failure
failed
ios universal examples Integrate
Running on macos-mojave-gstreamer-runner-zex-eccd-concurrent-0.shared via gst-gitlab-ci1.local...
Skipping Git repository setup
Skipping Git checkout
Skipping Git submodules setup
$ installer -pkg gstreamer-1.0-devel-*-ios-universal.pkg -target CurrentUserHomeDirectory -verbose
installer: Error - the package path specified was invalid: 'gstreamer-1.0-devel-*-ios-universal.pkg'.
Running after script...
$ rm -rf ${EXAMPLES_HOME}/gst-examples ${EXAMPLES_HOME}/gst-docs
ERROR: Job failed: Process exited with: 1. Reason was: ()