cmd/link/internal/ld: add missing Close

Noticed while working on issue 30488.

Change-Id: Ia3655e07c939d03925b3560eeba24c60e24c136c
Reviewed-on: https://go-review.googlesource.com/c/go/+/175917
Run-TryBot: Elias Naur <mail@eliasnaur.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/link/internal/ld/macho_combine_dwarf.go

@@ -99,6 +99,7 @@ func machoCombineDwarf(ctxt *Link, exef *os.File, exem *macho.File, dsym, outexe
 	if err != nil {
 		return err
 	}
+	defer dwarff.Close()
 	outf, err := os.Create(outexe)
 	if err != nil {
 		return err