MINOR: threads: Use __decl_hathreads to declare locks

This macro should be used to declare variables or struct members depending on
the USE_THREAD compile option. It avoids the encapsulation of such declarations
between #ifdef/#endif. It is used to declare all lock variables.
diff --git a/include/common/memory.h b/include/common/memory.h
index ee394f8..a5c0ec4 100644
--- a/include/common/memory.h
+++ b/include/common/memory.h
@@ -47,9 +47,7 @@
 
 struct pool_head {
 	void **free_list;
-#ifdef USE_THREAD
-	HA_SPINLOCK_T lock;     /* the spin lock */
-#endif
+	__decl_hathreads(HA_SPINLOCK_T lock); /* the spin lock */
 	struct list list;	/* list of all known pools */
 	unsigned int used;	/* how many chunks are currently in use */
 	unsigned int allocated;	/* how many chunks have been allocated */