modesetting: set gbm as dependency in meson build

Modifiers support needs gbm as a dependency. Without setting the dependency
included headers are not found reliably and the build might fail if the
headers are not placed in the default system include paths.

Signed-off-by: Roman Gilg <subdiff@gmail.com>
Reviewed-by: Adam Jackson <ajax@redhat.com>

hw/xfree86/drivers/modesetting/meson.build

@@ -19,6 +19,7 @@ shared_module(
         common_dep,
         udev_dep,
         libdrm_dep,
+        gbm_dep,
     ],
 
     install: true,