[Cpp] Duplicate using namespace lines.

[p7r0x7]

// Generated from WhixyLexer.g4 by ANTLR 4.13.2

#include "WhixyLexer.h"

using namespace antlr4;

using namespace Whixy;

using namespace antlr4;

This is a tiny tiny issue but the C++ generator is creating duplicate 'using namespace' lines.

[cppenjoy]

Fixed: #4745

[p7r0x7]

This can be closed upon merging, then!