meson: Define GST_USE_UNSTABLE_API in the build system

This way meson and autotools both do the same thing and we don't need to
modify the code after all.

tests/examples/mpegts/meson.build

@@ -3,5 +3,5 @@ executable('tsparser',
   install: true,
   include_directories : [configinc],
   dependencies : [gstmpegts_dep],
-  c_args : ['-DHAVE_CONFIG_H=1' ],
+  c_args : ['-DHAVE_CONFIG_H=1', '-DGST_USE_UNSTABLE_API' ],
 )

tests/examples/mpegts/ts-parser.c

@@ -30,11 +30,6 @@
 #include <glib-object.h>
 #include <glib/gprintf.h>
 #include <gst/gst.h>
-
-#ifndef GST_USE_UNSTABLE_API
-#define GST_USE_UNSTABLE_API
-#endif
-
 #include <gst/mpegts/mpegts.h>
 
 static void