interface: Silence warning from gcc version 8.

When compiling with gcc8 and -O2, the clever compiler complains:

   interface.c: In function ‘interface_ensure_tslabel’:
   interface.c:38:3: error: ‘strncpy’ output may be truncated copying 108 bytes from a string of length 108 [-Werror=stringop-truncation]

Even though this is a false positive, this patch silences the warning
by using memcpy instead of strncpy.

Signed-off-by: Richard Cochran <richardcochran@gmail.com>

interface.c

@@ -35,7 +35,7 @@ void interface_destroy(struct interface *iface)
 void interface_ensure_tslabel(struct interface *iface)
 {
 	if (!iface->ts_label[0]) {
-		strncpy(iface->ts_label, iface->name, MAX_IFNAME_SIZE);
+		memcpy(iface->ts_label, iface->name, MAX_IFNAME_SIZE);
 	}
 }