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/src/applet.c b/src/applet.c
index 0e550c2..1df6b09 100644
--- a/src/applet.c
+++ b/src/applet.c
@@ -22,10 +22,7 @@
 
 unsigned int nb_applets = 0;
 unsigned int applets_active_queue = 0;
-
-#ifdef USE_THREAD
-HA_SPINLOCK_T applet_active_lock;        /* spin lock related to applet active queue */
-#endif
+__decl_hathreads(HA_SPINLOCK_T applet_active_lock);  /* spin lock related to applet active queue */
 
 struct list applet_active_queue = LIST_HEAD_INIT(applet_active_queue);