meson: Fallback to 'opengl' when 'GL' is not found.

This patch adds 'opengl' as a fallback to 'GL' for dependency lookup, to
link with libglvnd configured without X11 support.

For OpenGL, libglvnd provides two pkg-config files: `gl.pc' with GLX
support while `opengl.pc' not.  When building without X11 support, the
former won't be installed.

src/meson.build

@@ -18,7 +18,7 @@ executable('Hyprland', src,
     xcb_dep,
 
     dependency('pixman-1'),
-    dependency('GL'),
+    dependency('GL', 'opengl'),
     dependency('threads')
   ],
   install : true