Add declarations to fix the build with MinGW 5.3.0 in C++11 mode

Fix the build with -std=c++11 (instead of -std=gnu++11) by providing even
more declarations for the functions wrongly excluded from MinGW headers in
"strict ANSI" mode ourselves.

src/common/filefn.cpp

@@ -83,6 +83,8 @@
 
 // TODO: Borland probably has _wgetcwd as well?
 #if defined(_MSC_VER) || defined(__MINGW32__)
+    wxDECL_FOR_STRICT_MINGW32(wchar_t*, _wgetcwd, (wchar_t*, int))
+
     #define HAVE_WGETCWD
 #endif