cmd/cgo: add missing formatting directive in error message

Fixes #15310.

Change-Id: I588b3c630a20a6878f7cd00f9af29b1dd8a4abf6
Reviewed-on: https://go-review.googlesource.com/22100
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/cgo/gcc.go

@@ -1009,7 +1009,7 @@ func (p *Package) rewriteRef(f *File) {
 			if r.Name.Kind == "var" {
 				expr = &ast.StarExpr{Star: (*r.Expr).Pos(), X: expr}
 			} else {
-				error_(r.Pos(), "only C variables allowed in selector expression", fixGo(r.Name.Go))
+				error_(r.Pos(), "only C variables allowed in selector expression %s", fixGo(r.Name.Go))
 			}
 
 		case "type":