BUILD: fd/threads: fix breakage build breakage without threads

The last fix for the volatile dereference made use of pl_deref_int()
which is unknown when building without threads. Let's simply open-code
it instead. No backport needed.
diff --git a/src/fd.c b/src/fd.c
index 87db0e9..d6c140c 100644
--- a/src/fd.c
+++ b/src/fd.c
@@ -265,7 +265,7 @@
 
 #else
 lock_self_next:
-	next = pl_deref_int(&fdtab[fd].cache.next);
+	next = ({ volatile int *next = &fdtab[fd].cache.next; *next; });
 	if (next == -2)
 		goto lock_self_next;
 	if (next <= -3)
@@ -273,7 +273,7 @@
 	if (unlikely(!HA_ATOMIC_CAS(&fdtab[fd].cache.next, &next, -2)))
 		goto lock_self_next;
 lock_self_prev:
-	prev = pl_deref_int(&fdtab[fd].cache.prev);
+	prev = ({ volatile int *prev = &fdtab[fd].cache.prev; *prev; });
 	if (prev == -2)
 		goto lock_self_prev;
 	if (unlikely(!HA_ATOMIC_CAS(&fdtab[fd].cache.prev, &prev, -2)))