Fix some harmless gcc 7 -Wimplicit-fallthrough warnings

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

src/common/list.cpp

@@ -51,6 +51,7 @@ bool wxListKey::operator==(wxListKeyValue value) const
             wxFAIL_MSG(wxT("bad key type."));
             // let compiler optimize the line above away in release build
             // by not putting return here...
+            wxFALLTHROUGH;
 
         case wxKEY_STRING:
             return *m_key.string == *value.string;