Fix VTY: add missing doc for 'log gsmtap [HOSTNAME]'

Add missing doc string for the HOSTNAME arg.

Added in commit aa00f99be2e4cc64ede20d8c9548b83054696581, this breaks the VTY
tests for openbsc and hence breaks our jenkins build jobs.

Change-Id: I734b22c950242541322e902887bf779c14ba10fd
diff --git a/src/vty/logging_vty.c b/src/vty/logging_vty.c
index cf3ec21..6004c47 100644
--- a/src/vty/logging_vty.c
+++ b/src/vty/logging_vty.c
@@ -501,7 +501,8 @@
 
 DEFUN(cfg_log_gsmtap, cfg_log_gsmtap_cmd,
 	"log gsmtap [HOSTNAME]",
-	LOG_STR "Logging via GSMTAP\n")
+	LOG_STR "Logging via GSMTAP\n"
+	"Host name to send the GSMTAP logging to (UDP port 4729)\n")
 {
 	const char *hostname = argv[0];
 	struct log_target *tgt;