egl: forward declare wl_resource for compatibility

Depending on what version of Mesa you have then eglQueryWaylandBuffer
may take a wl_buffer or wl_resource argument and the EGL header will
only forward declare the corresponding type.

The use of wl_buffer has been deprecated and so internally we assume
that eglQueryWaylandBuffer takes a wl_resource but for compatibility we
forward declare wl_resource in case we are building with EGL headers
that still use wl_buffer.

https://bugzilla.gnome.org/show_bug.cgi?id=710926

Reviewed-by: Neil Roberts <neil@linux.intel.com>
(cherry picked from commit 9bd1ee544667cfe7ecae479ec7f778446dd8f326)