[NaCL] Add missing include of <sys/mount.h>

This was previously included by nacl_io.h but
upstrean changes in NaCl removed this.

cocos2dx/platform/nacl/CCInstance.cpp

@@ -37,6 +37,7 @@ THE SOFTWARE.
 #include <pthread.h>
 #include <stdio.h>
 #include <sys/stat.h>
+#include <sys/mount.h>
 #include <unistd.h>
 
 USING_NS_CC;