kdb_sysctl_trap: suppress gcc -Warray-bounds

gcc diagnosed a dereference of 0x10 with -Warray-bounds, which is
entirely sensible, except that this is a deliberate trap.  Throw gcc off
with a volatile pointer.

Reviewed by:	kib (previous version)
Differential Revision:	https://reviews.freebsd.org/D45917

sys/kern/subr_kdb.c

@@ -238,7 +238,7 @@ static int
 kdb_sysctl_trap(SYSCTL_HANDLER_ARGS)
 {
 	int error, i;
-	int *addr = (int *)0x10;
+	int *volatile addr = (int *)0x10;
 
 	error = sysctl_wire_old_buffer(req, sizeof(int));
 	if (error == 0) {