unix: add //go:build line to gccgo_c.c

Change-Id: I425df52458cc53afb986a260a41d0b638bd5e6e8
Reviewed-on: https://go-review.googlesource.com/c/sys/+/460157
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Heschi Kreinick <heschi@google.com>
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
diff --git a/unix/gccgo_c.c b/unix/gccgo_c.c
index c4fce0e..f98a1c5 100644
--- a/unix/gccgo_c.c
+++ b/unix/gccgo_c.c
@@ -2,8 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
-// +build gccgo,!hurd
-// +build !aix,!hurd
+//go:build gccgo && !aix && !hurd
+// +build gccgo,!aix,!hurd
 #include <errno.h>
 #include <stdint.h>