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 <[email protected]>
Reviewed-by: Heschi Kreinick <[email protected]>
Auto-Submit: Tobias Klauser <[email protected]>
Run-TryBot: Tobias Klauser <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>

Author: tklauser

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>