BUILD: debug: Add braces to if statement calling only CHECK_IF()

In src/ev_epoll.c, a CHECK_IF() is guarded by an if statement. So, when the
macro is empty, GCC (at least 11.3.1) is not happy because there is an if
statement with an empty body without braces... It is handled by
"-Wempty-body" option.

So, braces are added and GCC is now happy.

No backport needed.
diff --git a/src/ev_epoll.c b/src/ev_epoll.c
index 884c038..f35e1c2 100644
--- a/src/ev_epoll.c
+++ b/src/ev_epoll.c
@@ -57,8 +57,9 @@
 		 * wrong thread or during startup, which is what we're checking
 		 * for. Regardless, it is not a problem to do so.
 		 */
-		if (unlikely(!(global.mode & MODE_STARTING)))
+		if (unlikely(!(global.mode & MODE_STARTING))) {
 			CHECK_IF(tgid != tgrp && !thread_isolated());
+		}
 
 		for (i = ha_tgroup_info[tgrp-1].base; i < ha_tgroup_info[tgrp-1].base + ha_tgroup_info[tgrp-1].count; i++)
 			if (m & ha_thread_info[i].ltid_bit)