Provide empty wxDECL_FOR_STRICT_MINGW32() in non-MinGW case.

Fix the build after aa30a2f97a which forgot to
define wxDECL_FOR_STRICT_MINGW32() macro when MinGW was not used at all.

include/wx/platform.h

@@ -459,6 +459,7 @@
 #    define wxCHECK_W32API_VERSION(maj, min) (0)
 #    undef wxCHECK_MINGW32_VERSION
 #    define wxCHECK_MINGW32_VERSION( major, minor ) (0)
+#    define wxDECL_FOR_STRICT_MINGW32(rettype, func, params)
 #endif