lib: use constant to replace magic value for length of quagga_timestamp

Signed-off-by: Christian Franke <chris@opensourcerouting.org>
diff --git a/lib/vty.c b/lib/vty.c
index 3891661..bf259fc 100644
--- a/lib/vty.c
+++ b/lib/vty.c
@@ -210,7 +210,7 @@
 void
 vty_time_print (struct vty *vty, int cr)
 {
-  char buf [25];
+  char buf[QUAGGA_TIMESTAMP_LEN];
   
   if (quagga_timestamp(0, buf, sizeof(buf)) == 0)
     {