libs: remove unneeded headers

libs: remove unneeded headers

Since gstvaapidisplay_glx.h do not expose gl.h/glx.h structures, it is not
required to include them in the header. It is not also required to include
them in gstvaapidisplay_glx.c, since gstvaapiutils_glx.h includes them and
exposes their structures (e.g. GLXPixmap).

Nonetheless, glext.h neither glxext.h are required to include, they are
already included conditionally by gl.h and glx.h, respectively.

Signed-off-by: Víctor Manuel Jáquez Leal <victorx.jaquez@intel.com>
https://bugzilla.gnome.org/show_bug.cgi?id=757577