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/server.h b/include/types/server.h
index 76225f7..adedca4 100644
--- a/include/types/server.h
+++ b/include/types/server.h
@@ -286,10 +286,7 @@
 		struct sample_expr *sni;        /* sample expression for SNI */
 	} ssl_ctx;
 #endif
-
-#ifdef USE_THREAD
-	HA_SPINLOCK_T lock;
-#endif
+	__decl_hathreads(HA_SPINLOCK_T lock);
 	struct {
 		const char *file;		/* file where the section appears */
 		int line;			/* line where the section appears */