Fix a warning from GCC - 'ALIAS' was unhandled in the switch stmt.

simgear/props/condition.cxx

@@ -213,8 +213,10 @@ doComparison (const SGPropertyNode * left, const SGPropertyNode *right)
       return SGComparisonCondition::EQUALS;
     break;
   }
-  }
+  default:
     throw sg_exception("condition: unrecognized node type in comparison");
+  }
+  
   return 0;
 }