go/gcimporter15: add missing argument to error message

Backport of Change 19641

Change-Id: Ic386cc8ecc006a2cd96c5ac3ff00f3d314873fd3
Reviewed-on: https://go-review.googlesource.com/19643
Reviewed-by: Robert Griesemer <gri@golang.org>

go/gcimporter15/gcimporter.go

@@ -396,7 +396,7 @@ func (p *parser) getPkg(id, name string) *types.Package {
 		if pname := pkg.Name(); pname == "" {
 			setName(pkg, name)
 		} else if pname != name {
-			p.errorf("%s package name mismatch: %s (given) vs %s (expected)", pname, name)
+			p.errorf("%s package name mismatch: %s (given) vs %s (expected)", id, pname, name)
 		}
 	}
 	return pkg