BUILD: activity: do not include malloc.h

It doesn't exist on MacOS and broke the build. We don't need it as it's
already included by compat.h when relevant. No backport is needed.
diff --git a/src/activity.c b/src/activity.c
index 3f69e44..df8b9bd 100644
--- a/src/activity.c
+++ b/src/activity.c
@@ -10,7 +10,6 @@
  *
  */
 
-#include <malloc.h>
 #include <haproxy/activity-t.h>
 #include <haproxy/api.h>
 #include <haproxy/cfgparse.h>