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/types/peers.h b/include/types/peers.h
index 2fc7435..58c8c4e 100644
--- a/include/types/peers.h
+++ b/include/types/peers.h
@@ -67,9 +67,7 @@
 	struct shared_table *remote_table;
 	struct shared_table *last_local_table;
 	struct shared_table *tables;
-#ifdef USE_THREAD
-	HA_SPINLOCK_T lock;	 /* lock used to handle this peer section */
-#endif
+	__decl_hathreads(HA_SPINLOCK_T lock); /* lock used to handle this peer section */
 	struct peer *next;	  /* next peer in the list */
 };