win32: Fix a couple of build warnings

* clutter-backend-win32 had an unused variable

* clutter-event-win32 was using the abs function without including
  stdlib.h

clutter/win32/clutter-backend-win32.c

@@ -586,8 +586,6 @@ clutter_backend_win32_class_init (ClutterBackendWin32Class *klass)
 static void
 clutter_backend_win32_init (ClutterBackendWin32 *backend_win32)
 {
-  ClutterBackend *backend = CLUTTER_BACKEND (backend_win32);
-
   backend_win32->gl_context         = NULL;
   backend_win32->invisible_cursor   = NULL;
 

clutter/win32/clutter-event-win32.c

@@ -40,6 +40,7 @@
 #include <glib.h>
 #include <windows.h>
 #include <windowsx.h>
+#include <stdlib.h>
 
 typedef struct _ClutterEventSource      ClutterEventSource;