net: comment duplicate constant

Change-Id: If5a4d8eff4e51d72fb9dc1d5db2bfe674ec5753b
Reviewed-on: https://go-review.googlesource.com/c/go/+/176717Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/net/tcpsockopt_darwin.go

@@ -10,6 +10,7 @@ import (
 	"time"
 )
 
+// syscall.TCP_KEEPINTVL is missing on some darwin architectures.
 const sysTCP_KEEPINTVL = 0x101
 
 func setKeepAlivePeriod(fd *netFD, d time.Duration) error {