CLEANUP: thread: rename __decl_hathreads() to __decl_thread()

I can never figure whether it takes an "s" or not, and in the end it's
better if it matches the file's naming, so let's call it "__decl_thread".
diff --git a/src/memory.c b/src/memory.c
index 0bca712..a0e57c7 100644
--- a/src/memory.c
+++ b/src/memory.c
@@ -600,7 +600,7 @@
 #define MEM_FAIL_MAX_STR 128
 static int mem_fail_cur_idx;
 static char mem_fail_str[MEM_FAIL_MAX_CHAR * MEM_FAIL_MAX_STR];
-__decl_hathreads(static HA_SPINLOCK_T mem_fail_lock);
+__decl_thread(static HA_SPINLOCK_T mem_fail_lock);
 
 int mem_should_fail(const struct pool_head *pool)
 {