unix: add missing HCI_CHANNEL_LOGGING constant

For more information see: https://git.kernel.org/pub/scm/linux/kernel/git/bluetooth/bluetooth.git/commit/?id=ac71494934c475e3f51e5e3e64a12f57618d82a4

Change-Id: If67f020c5a4ed64726d6ef35ba03d5ed100cedc4
Reviewed-on: https://go-review.googlesource.com/c/sys/+/203610
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
diff --git a/unix/bluetooth_linux.go b/unix/bluetooth_linux.go
index 6e32296..a178a61 100644
--- a/unix/bluetooth_linux.go
+++ b/unix/bluetooth_linux.go
@@ -23,6 +23,7 @@
 	HCI_CHANNEL_USER    = 1
 	HCI_CHANNEL_MONITOR = 2
 	HCI_CHANNEL_CONTROL = 3
+	HCI_CHANNEL_LOGGING = 4
 )
 
 // Socketoption Level