cmake: fix tests linking

Flatbuffers is header only library! What we link with (builded in `external/flatbuffers/CMakeLists.txt`) is a `flatc` - flatbuffers protocol compiler. As a result of link with that "library" function `main` was injected in cocos2d.a and on windows (mingw) cpp-tests.exe, cpp-empty-test.exe and others... became... flatc command line executables ;-)

Fix this by not link with flatbuffers target, only use include directory for it.

It is quick fix, I think that actually we need modify also `external/flatbuffers/CMakeLists.txt` to do one of:
* Do not build anything... if we do not need provide `flatc` utility to users
* Build executable... if we do need provide `flatc` utility to users

Perhaps this decision can be taken by people who use this feature.