meson: define inc_gbm as empty if not otherwise assigned

Otherwise this could be undefined in the egl directory.

Signed-off-by: Dylan Baker <dylan.c.baker@intel.com>
Acked-by: Eric Engestrom <eric.engestrom@imgtec.com>

src/meson.build

@@ -67,6 +67,8 @@ subdir('loader')
 subdir('glx')
 if with_gbm
   subdir('gbm')
+else
+  inc_gbm = []
 endif
 if with_egl
   subdir('egl')