meson: gl: remove non-headers from gl_prototype_headers

This made the meson build install those files, while they aren't
installed with the autotools build.

https://bugzilla.gnome.org/show_bug.cgi?id=796274
diff --git a/gst-libs/gst/gl/meson.build b/gst-libs/gst/gl/meson.build
index 4ab5c57..a483066 100644
--- a/gst-libs/gst/gl/meson.build
+++ b/gst-libs/gst/gl/meson.build
@@ -75,10 +75,8 @@
   'glprototypes/gles.h',
   'glprototypes/gstgl_compat.h',
   'glprototypes/gstgl_gles2compat.h',
-  'glprototypes/Makefile.am',
   'glprototypes/opengl.h',
   'glprototypes/query.h',
-  'glprototypes/README',
   'glprototypes/shaders.h',
   'glprototypes/sync.h',
   'glprototypes/vao.h',