Fix declaration of lstrcmp/lstrcmpi in windows.d

According to MSDN (http://msdn.microsoft.com/en-us/library/windows/desktop/ms647488(v=vs.85).aspx), lstrcmp and lstrcmpi
follow the usual pattern of being macros expanding to either the ANSI (lstrcmpA) or Unicode (lstrcmpW).
The current declaration in windows.d misses the postfix A/W. This causes compile errors with ldc because the
mangled names are identical.
Merge pull request #874 from redstar/lstrcmp

Fix declaration of lstrcmp/lstrcmpi in windows.d