Added missing assert.h to util.h.

This was only causing compiler errors on certain compiler configurations.

Change-Id: I110fb8c896f74aae7ef739e9a29c636393dbbde2
Tested: on Linux and Windows.

include/flatbuffers/util.h

@@ -22,6 +22,7 @@
 #include <string>
 #include <sstream>
 #include <stdlib.h>
+#include <assert.h>
 #ifdef _WIN32
 #include <direct.h>
 #else