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

@@ -28,6 +28,7 @@
 
 #include <meta/meta-backend.h>
 #include <meta/util.h>
+#include <math.h>
 
 #include <cogl/cogl.h>
 #include <clutter/clutter.h>