plugins: include main gstgl header

Instead including particular gstgl header files in a header file
that doesn't export a gstgl symbol, the main gstgl header file is
included in gstvaapipluginutil.c where the symbols are used.

https://bugzilla.gnome.org/show_bug.cgi?id=786597

gst/vaapi/gstvaapipluginbase.h

@@ -31,10 +31,6 @@
 #include <gst/video/gstvideosink.h>
 #include <gst/vaapi/gstvaapidisplay.h>
 
-#if USE_GST_GL_HELPERS
-# include <gst/gl/gstglcontext.h>
-#endif
-
 G_BEGIN_DECLS
 
 typedef struct _GstVaapiPluginBase GstVaapiPluginBase;

gst/vaapi/gstvaapipluginutil.c

@@ -39,6 +39,9 @@
 #if USE_WAYLAND
 # include <gst/vaapi/gstvaapidisplay_wayland.h>
 #endif
+#if USE_GST_GL_HELPERS
+# include <gst/gl/gl.h>
+#endif
 #include "gstvaapipluginutil.h"
 #include "gstvaapipluginbase.h"