Fix implicit int and incompatible pointer errors
Fixes the following errors:
vinagre/vinagre-tube-handler.c:115:17: error: assignment to ‘TpChannel *’ {aka ‘struct _TpChannel *’} from incompatible pointer type ‘void **’ [-Wincompatible-pointer-types]
115 | priv->channel = g_object_ref (channel);
| ^
plugins/rdp/vinagre-rdp-plugin.c:237:12: error: type defaults to ‘int’ in declaration of ‘dontoptimiseaway’ [-Wimplicit-int]
237 | volatile dontoptimiseaway = vinagre_rdp_plugin_get_type ();
| ^~~~~~~~~~~~~~~~