gnome3.vinagre: fix build

Currently, vinagre fails to build with the following message:

vinagre/vinagre-utils.c: In function 'vinagre_utils_request_credential':
vinagre/vinagre-utils.c:686:2: error: format not a string literal, argument types not checked [-Werror=format-nonliteral]
  _tmp12_ = g_strdup_printf (_tmp10_, _tmp11_);
  ^~~~~~~

vinagre-utils.c seems to be generated from vinagre-utils.vala. I
couldn't find anything weird in here, so let's disable
-Werror=format-nonliteral for now as done elsewhere, too.