Fix compiler warnings.

meta-cursor-renderer.c is already fixed upstream in a later release:
https://git.gnome.org/browse/mutter/commit?id=ff312b69585cb0ec9f53483baa47678db8d44306

Other two files need to be submitted upstream still.

--- mutter-3.18.0/src/backends/meta-cursor-renderer.c	Sun Oct  4 19:47:56 2015
+++ mutter-3.18.0/src/backends/meta-cursor-renderer.c	Sun Oct  4 19:48:42 2015
@@ -26,6 +26,7 @@
 
 #include "meta-cursor-renderer.h"
 
+#include <math.h> 
 #include <meta/meta-backend.h>
 #include <meta/util.h>
 
--- mutter-3.18.0/src/core/window.c	Mon Sep 21 12:48:51 2015
+++ mutter-3.18.0/src/core/window.c	Tue Jan  5 13:34:16 2016
@@ -3488,7 +3488,7 @@
     {
       MetaMonitorInfo *info = &window->screen->monitor_infos[i];
 
-      if (info->winsys_id == winsys_id)
+      if ((size_t)info->winsys_id == winsys_id)
         return info;
     }
 
@@ -3697,7 +3697,7 @@
 
   meta_window_update_monitor (window, flags & META_MOVE_RESIZE_USER_ACTION);
 
-  if (old_output_winsys_id != window->monitor->winsys_id &&
+  if (old_output_winsys_id != (size_t) window->monitor->winsys_id &&
       flags & META_MOVE_RESIZE_MOVE_ACTION && flags & META_MOVE_RESIZE_USER_ACTION)
     window->preferred_output_winsys_id = window->monitor->winsys_id;

--- mutter-3.18.1/src/x11/session.c	Tue Jan  5 13:40:50 2016
+++ mutter-3.18.1/src/x11/session.c	Tue Jan  5 13:41:09 2016
@@ -336,7 +336,7 @@
     prop3val.value = &hint;
     prop3val.length = 1;
 
-    sprintf (pid, "%d", getpid ());
+    sprintf (pid, "%ld", (long)getpid ());
     prop4.name = (char *)SmProcessID;
     prop4.type = (char *)SmARRAY8;
     prop4.num_vals = 1;