meson: define HAVE_VIDEO_EXT if OMX_VideoExt.h is present

Meson build wasn't defining the HAVE_VIDEO_EXT like configure.ac does.

https://bugzilla.gnome.org/show_bug.cgi?id=783346

meson.build

@@ -168,6 +168,9 @@ have_external_omx = cc.has_header(
     args : gst_omx_args,
     required : false)
 extra_video_headers = ''
+# Our internal OpenMAX IL headers have OMX_VideoExt.h
+have_video_ext = true
+
 if have_external_omx
   have_video_ext = cc.has_header (
       'OMX_VideoExt.h',
@@ -179,6 +182,10 @@ if have_external_omx
   endif
 endif
 
+if have_video_ext
+  cdata.set ('HAVE_VIDEO_EXT', 1)
+endif
+
 have_omx_vp8 = cc.has_header_symbol(
     'OMX_Video.h',
     'OMX_VIDEO_CodingVP8',