meson: fix private libs when building without glx

I noticed that the generated pkg-config files will include
glx and x11 dependencies even when x11 isn't a selected platform.

This fixes the private libs and was tested by building kmscube

V2:
  - check if gallium-xlib is being used for glx

Fixes: 108d257a "meson: build libEGL"
Reviewed-by: Dylan Baker <dylan@pnwbakers.com>
Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>
(cherry picked from commit 4cfc4cef)