Add older style build comment

To compatible with older Go version on some cloud environment , such as Google App Engine using 1.16

sockopts_other.go

 //go:build !linux
+// +build !linux
 
 package gost
 
@@ -8,4 +9,4 @@ func setSocketMark(fd int, value int) (e error) {
 
 func setSocketInterface(fd int, value string) (e error) {
 	return nil
-}
+}
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