[bwn] add opt_wlan.h / opt_bwn.h for build-time control

This is a separate commit as I have local changes to bwn for now and
this needs a manual merge/commit.

Sponsored by:	Palm Springs

sys/dev/bwn/if_bwn.c

@@ -34,6 +34,9 @@ __FBSDID("$FreeBSD$");
  * The Broadcom Wireless LAN controller driver.
  */
 
+#include "opt_bwn.h"
+#include "opt_wlan.h"
+
 #include <sys/param.h>
 #include <sys/systm.h>
 #include <sys/kernel.h>