Don't leak strings in merge declarations

Introduced by the reentrant scanner commit.

Signed-off-by: Daniel Stone <daniel@fooishbar.org>

src/xkbcomp/xkbparse.y

@@ -337,6 +337,7 @@ Decl		:	OptMergeMode VarDecl
 			    else {
 				$$= &IncludeCreate($2,$1)->common;
 			    }
+                            free($2);
                         }
 		;