cmd/compile: fix format error

Found by pending CL to make cmd/vet auto-detect printf wrappers.

Change-Id: I6b5ba8f9c301dd2d7086c152cf2e54a68b012208
Reviewed-on: https://go-review.googlesource.com/109345
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/compile/internal/gc/iexport.go

@@ -1380,7 +1380,7 @@ func (w *exportWriter) localIdent(s *types.Sym, v int32) {
 	}
 
 	if i := strings.LastIndex(name, "."); i >= 0 {
-		Fatalf("unexpected dot in identifier:", name)
+		Fatalf("unexpected dot in identifier: %v", name)
 	}
 
 	if v > 0 {