Improve PR #331 (Fixed a C++ bool variable to WINAPI BOOL ...)

Improved [PR #331](https://github.com/PCSX2/pcsx2/pull/331), since there is no guarantee that C++ "true" and WINAPI "TRUE" are the same value in the future.

Here is a diff of before PR #331 and this change.
https://github.com/tadatada/pcsx2/commit/86d2ba19002df44876c21cb505ed2c7ef917087c