Restore default behaviour for stamps

@mdosch discovered that we now don't print `:` for messages from others
anymore.

Until
https://github.com/profanity-im/profanity/pull/1663#issuecomment-1114625151
is fixed lets restore the default behaviour to always add `:` in
`_win_print_internal()`.

The result will be that a stamp set to `-->` will also look like `-->:`.

src/config/preferences.c

@@ -2292,7 +2292,7 @@ _get_default_string(preference_t pref)
         return editor ? editor : "vim";
     }
     case PREF_OUTGOING_STAMP:
-        return "me:";
+        return "me";
     case PREF_INCOMING_STAMP:
         return NULL;
     case PREF_URL_SAVE_CMD: