xmm: fix warnings with -Wswitch-enum

xmm/mm-shared-xmm.c: In function ‘gps_engine_start’:
  xmm/mm-shared-xmm.c:1068:5: error: enumeration value ‘GPS_ENGINE_STATE_OFF’ not handled in switch [-Werror=switch-enum]
   1068 |     switch (state) {
        |     ^~~~~~

plugins/xmm/mm-shared-xmm.c

@@ -1078,6 +1078,7 @@ gps_engine_start (GTask *task)
             transport_protocol = 1;
             pos_mode = 2;
             break;
+        case GPS_ENGINE_STATE_OFF:
         default:
             g_assert_not_reached ();
             break;