handler: avoid unnamed union

Unnamed unions appeared only in C11 standard. Don't use them to
increase portability.

src/common.h

@@ -130,7 +130,7 @@ struct _xmpp_handlist_t {
             char *name;
             char *type;
         };
-    };
+    } u;
 };
 
 #define MAX_DOMAIN_LEN 256