BUILD: kqueue/threads: Add test on MAX_THREADS to avoid warnings when complied without threads

This is the same patch than the previous one ("BUILD: epoll/threads: Add test on
MAX_THREADS to avoid warnings when complied without threads ").

It should be backported in 1.8 with the commit 7a2364d4 ("BUG/MEDIUM:
kqueue/threads: use one kqueue_fd per thread").
diff --git a/src/ev_kqueue.c b/src/ev_kqueue.c
index d58bc4a..86731af 100644
--- a/src/ev_kqueue.c
+++ b/src/ev_kqueue.c
@@ -161,7 +161,7 @@
 	if (kev == NULL)
 		goto fail_alloc;
 
-	if (tid) {
+	if (MAX_THREADS > 1 && tid) {
 		kqueue_fd[tid] = kqueue();
 		if (kqueue_fd[tid] < 0)
 			goto fail_fd;
@@ -184,7 +184,7 @@
 
 static void deinit_kqueue_per_thread()
 {
-	if (tid)
+	if (MAX_THREADS > 1 && tid)
 		close(kqueue_fd[tid]);
 
 	free(kev);