webkitgtk: Add patch for properly picking GL flags when building

This solves build failures caused by WebKit trying to include X11 headers
when support for X11 is disabled in Mesa3D. A common situation is when
configuring both GTK+ and WebKitGTK+ only with Wayland support.

Once the fix for https://bugs.webkit.org/show_bug.cgi?id=175125

 makes it
into a release, the patch can be dropped.

Signed-off-by: Adrian Perez de Castro <aperez@igalia.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>