tty_pts: don't rely on tty header pollution for sys/mutex.h

tty_pts.c relies on sys/tty.h for sys/mutex.h. Include it directly instead
of relying on this pollution to ease the diff for anyone that wants to try
converting the tty lock to anything other than a mutex.

sys/kern/tty_pts.c

@@ -50,6 +50,7 @@ __FBSDID("$FreeBSD$");
 #include <sys/kernel.h>
 #include <sys/limits.h>
 #include <sys/malloc.h>
+#include <sys/mutex.h>
 #include <sys/poll.h>
 #include <sys/proc.h>
 #include <sys/racct.h>