BUILD: tree-wide: add several missing activity.h
A number of files currently access activity counters but rely on their
definitions to be inherited from other files (task.c, backend.c hlua.c,
sock.c, pool.c, stats.c, fd.c).
diff --git a/src/backend.c b/src/backend.c
index 43642a3..6ec85ae 100644
--- a/src/backend.c
+++ b/src/backend.c
@@ -21,8 +21,9 @@
#include <import/ebmbtree.h>
-#include <haproxy/acl.h>
#include <haproxy/api.h>
+#include <haproxy/acl.h>
+#include <haproxy/activity.h>
#include <haproxy/arg.h>
#include <haproxy/backend.h>
#include <haproxy/channel.h>