freebsd_network/malloc: include kernel/heap.h

This header was not included when Haiku is built with KDEBUG_LEVEL=0,
causing build failures.

Change-Id: I690064f7c1a9818056a394e2655811775c4cc554
Reviewed-on: https://review.haiku-os.org/c/haiku/+/2360
Reviewed-by: waddlesplash <waddlesplash@gmail.com>

src/libs/compat/freebsd_network/malloc.cpp

@@ -13,6 +13,7 @@ extern "C" {
 
 #include <util/BitUtils.h>
 
+#include <kernel/heap.h>
 #include <kernel/vm/vm.h>