Fix build with MinGW 5.3 in debug mode

Due to a bug in MinGW (see https://sourceforge.net/p/mingw/bugs/2322/),
_stricmp() and _strnicmp() declarations are not visible when compiling without
optimizations. Work around this by declaring them ourselves.

See #17762.

include/wx/platform.h

@@ -358,6 +358,7 @@
 #    define wxCHECK_W32API_VERSION(maj, min) (0)
 #    undef wxCHECK_MINGW32_VERSION
 #    define wxCHECK_MINGW32_VERSION( major, minor ) (0)
+#    define wxDECL_FOR_MINGW32_ALWAYS(rettype, func, params)
 #    define wxDECL_FOR_STRICT_MINGW32(rettype, func, params)
 #endif