path/filepath: use new style deprecation message

Change-Id: I242a8960583e333f372929aad4adb8efbe441cd4
Reviewed-on: https://go-review.googlesource.com/28413
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Jaana Burcu Dogan <jbd@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/path/filepath/path_unix.go

@@ -20,6 +20,8 @@ func volumeNameLen(path string) int {
 }
 
 // HasPrefix exists for historical compatibility and should not be used.
+//
+// Deprecated: Use strings.HasPrefix instead.
 func HasPrefix(p, prefix string) bool {
 	return strings.HasPrefix(p, prefix)
 }