Makefile: Add httpserver rule

The httpserver is part of the compilation of the mgmt.

Signed-off-by: Amnon Heiman <amnon@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>

modules/mgmt/Makefile

-module:
+module: httpserver
+
+httpserver:
+	make -C ${OSV_BASE}/mgmt/httpserver