cmd/gomobile: fix build

Change-Id: Ia4f09fbc56ca4f0c31ef075049b7854442b0155e
Reviewed-on: https://go-review.googlesource.com/12644
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>

cmd/gomobile/bind.go

@@ -99,6 +99,8 @@ func runBind(cmd *command) error {
 		return goAndroidBind(pkg)
 	case "ios":
 		return goIOSBind(pkg)
+	default:
+		return fmt.Errorf(`unknown -target, %q.`, buildTarget)
 	}
 }