syscall: fix build directive in types_linux.go

The rule is that build directives can be preceded only
by blank lines and other line comments, not /* */ comments.

R=golang-dev, adg, bradfitz
CC=golang-dev
https://golang.org/cl/5619045

src/pkg/syscall/types_linux.go

@@ -2,9 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-/*
-Input to cgo -godefs.  See also mkerrors.sh and mkall.sh
-*/
+// Input to cgo -godefs.  See also mkerrors.sh and mkall.sh
 
 // +build ignore