fbsd-nat: Fix build failure with GCC 12

A user pointed out that the build failed on FreeBSD/amd64 with my last
commit.  The problem is that I'm not using the proper way to tell the
compiler that the variable has been "used".  This patch fixes this issue
as suggested by John.  Pushed as obvious.

Tested both on FreeBSD/amd64 and FreeBSD/aarch64 by rebuilding.

Suggested-By: John Baldwin <jhb@FreeBSD.org>

gdb/fbsd-nat.c

@@ -2094,7 +2094,7 @@ fbsd_nat_target::detach (inferior *inf, int from_tty)
 		  }
 #else
 		  /* pacify gcc  */
-		  wptid = (void) null_ptid;
+		  (void) wptid;
 #endif
 		  sig = 0;
 		  break;