[mutter] build: Fix build when native backend is disabled

[Ting-Wei Lan <lantw src gnome org>]

commit 31eafba93af28fa404c14a7ff8b4d90f1e6eba68
Author: Ting-Wei Lan <lantw src gnome org>
Date:   Sun Jan 7 03:28:21 2018 +0800

    build: Fix build when native backend is disabled
    
    https://bugzilla.gnome.org/show_bug.cgi?id=792281

 src/wayland/meta-wayland-keyboard.c |    2 ++
 1 files changed, 2 insertions(+), 0 deletions(-)
---
diff --git a/src/wayland/meta-wayland-keyboard.c b/src/wayland/meta-wayland-keyboard.c
index 5b90b2a..f5101ab 100644
--- a/src/wayland/meta-wayland-keyboard.c
+++ b/src/wayland/meta-wayland-keyboard.c
@@ -442,6 +442,7 @@ maybe_restore_numlock_state (MetaWaylandKeyboard *keyboard)
 static void
 maybe_save_numlock_state (MetaWaylandKeyboard *keyboard)
 {
+#ifdef HAVE_NATIVE_BACKEND
   MetaWaylandXkbInfo *xkb_info = &keyboard->xkb_info;
   GsdKeyboardNumLockState numlock_state;
   int numlock_active;
@@ -474,6 +475,7 @@ maybe_save_numlock_state (MetaWaylandKeyboard *keyboard)
       break;
     }
   g_settings_set_enum (keyboard->gsd_settings, "numlock-state", numlock_state);
+#endif
 }
 
 static void