Improve recompile speed: Don't include git_rev.h/git_revision.h in system.h (leads to excessive recompiles)
[Win32] Remove condition `!defined(_LIB)` for git_revision.h inclusion

XBMC seems to compile and display a git revision with or without this
condition.

This reverts the change to PlatformInclude.h (now GitRevision.cpp) in b4952cf