Fix some harmless gcc 7 -Wimplicit-fallthrough warnings

Add wxFALLTHROUGH to avoid the warnings where fall-through is actually
desired.

include/wx/private/wxprintf.h

@@ -302,6 +302,7 @@ bool wxPrintfConvSpec<CharType>::Parse(const CharType *format)
                     break;
                 }
                 // else: fall-through, 'I' is MSVC equivalent of C99 'z'
+                wxFALLTHROUGH;
 #endif      // __WINDOWS__
 
             case wxT('z'):