ui/gl: opengl doesn't require PIXMAN

The QEMU fallback covers the requirements. We still need the flags of
header inclusion with CONFIG_PIXMAN.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>

ui/meson.build

@@ -60,8 +60,8 @@ endif
 system_ss.add(opengl)
 if opengl.found()
   opengl_ss = ss.source_set()
-  opengl_ss.add(gbm)
-  opengl_ss.add(when: [opengl, pixman],
+  opengl_ss.add(gbm, pixman)
+  opengl_ss.add(when: [opengl],
                if_true: files('shader.c', 'console-gl.c', 'egl-helpers.c', 'egl-context.c'))
   ui_modules += {'opengl' : opengl_ss}
 endif