gc: fix bug introduced earlier

Apparently l and $1 were the same register on Linux.
On the other systems, the compiler caught it.

R=ken2
CC=golang-dev
https://golang.org/cl/5654061

src/cmd/gc/go.y

@@ -1422,6 +1422,7 @@ structdcl:
 		NodeList *l;
 
 		Node *n;
+		l = $1;
 		if(l != nil && l->next == nil && l->n == nil) {
 			// ? symbol, during import
 			n = $2;