[mutter] fix warning due implicit declaration of function

[Victor Toso de Carvalho <victortoso src gnome org>]

commit ff312b69585cb0ec9f53483baa47678db8d44306
Author: Victor Toso <victortoso redhat com>
Date:   Tue Apr 19 11:33:14 2016 +0200

    fix warning due implicit declaration of function
    
    error: implicit declaration of function
      .x = (int)roundf (priv->current_x - (hot_x * texture_scale)),
                ^
    
    https://bugzilla.gnome.org/show_bug.cgi?id=765252

 src/backends/meta-cursor-renderer.c |    1 +
 1 files changed, 1 insertions(+), 0 deletions(-)
---
diff --git a/src/backends/meta-cursor-renderer.c b/src/backends/meta-cursor-renderer.c
index 48cd239..ca99c5d 100644
--- a/src/backends/meta-cursor-renderer.c
+++ b/src/backends/meta-cursor-renderer.c
@@ -28,6 +28,7 @@
 
 #include <meta/meta-backend.h>
 #include <meta/util.h>
+#include <math.h>
 
 #include <cogl/cogl.h>
 #include <clutter/clutter.h>