Fix failing build on mageia v6 x64 linux.

There was an error about undeclared malloc()+free().

platform/x11/godot_x11.cpp

@@ -28,6 +28,7 @@
 /*************************************************************************/
 #include <unistd.h>
 #include <limits.h>
+#include <stdlib.h>
 
 #include "main/main.h"
 #include "os_x11.h"