all: add //go:build lines

They're automatically added by Go 1.17 source formatters. This change
adds them all now.

Signed-off-by: Elias Naur <mail@eliasnaur.com>

gpu/headless/headless_egl.go

@@ -1,5 +1,6 @@
 // SPDX-License-Identifier: Unlicense OR MIT
 
+//go:build linux || freebsd || windows || openbsd
 // +build linux freebsd windows openbsd
 
 package headless

gpu/headless/headless_gl.go

@@ -1,5 +1,6 @@
 // SPDX-License-Identifier: Unlicense OR MIT
 
+//go:build !windows
 // +build !windows
 
 package headless