[metacity] vulkan: fix a build warning

[Alberts Muktupāvels <muktupavels src gnome org>]

commit 5a204b8da4d615ee631fd595af3b6357e3b85303
Author: Alberts Muktupāvels <alberts muktupavels gmail com>
Date:   Mon Jul 3 15:15:19 2017 +0300

    vulkan: fix a build warning

 src/compositor/meta-compositor-vulkan.c |    2 ++
 1 files changed, 2 insertions(+), 0 deletions(-)
---
diff --git a/src/compositor/meta-compositor-vulkan.c b/src/compositor/meta-compositor-vulkan.c
index dfa9170..9a507f0 100644
--- a/src/compositor/meta-compositor-vulkan.c
+++ b/src/compositor/meta-compositor-vulkan.c
@@ -1188,6 +1188,7 @@ meta_compositor_vulkan_finalize (GObject *object)
   G_OBJECT_CLASS (meta_compositor_vulkan_parent_class)->finalize (object);
 }
 
+#ifdef HAVE_VULKAN
 static gboolean
 not_implemented_cb (MetaCompositorVulkan *vulkan)
 {
@@ -1204,6 +1205,7 @@ not_implemented_cb (MetaCompositorVulkan *vulkan)
 
   return G_SOURCE_REMOVE;
 }
+#endif
 
 static gboolean
 meta_compositor_vulkan_manage (MetaCompositor  *compositor,