xspawn: Test the right macro for posix_spawn_..._addfchdir() support

This should have been benign (just a performance regression), but
FreeBSD has a bug that causes the fork()-based fallback to lock up when
ASAN is enabled, due to fork() and dl_iterate_phdr().  This is not a
complete workaround for that issue, since the fork() fallback may be
used for other reasons.

Fixes: 29ddac2 ("config: Check for posix_spawn_file_actions_addfchdir{,_np}()")

src/xspawn.c

@@ -228,7 +228,7 @@ int bfs_spawn_addfchdir(struct bfs_spawn *ctx, int fd) {
 #  define BFS_POSIX_SPAWN_ADDFCHDIR posix_spawn_file_actions_addfchdir_np
 #endif
 
-#if _POSIX_SPAWN > 0 && defined(BFS_POSIX_SPAWN_FCHDIR)
+#if _POSIX_SPAWN > 0 && defined(BFS_POSIX_SPAWN_ADDFCHDIR)
 	if (ctx->flags & BFS_SPAWN_USE_POSIX) {
 		errno = BFS_POSIX_SPAWN_ADDFCHDIR(&ctx->actions, fd);
 		if (errno != 0) {