httpserver: compile using boost headers matching libraries we link with

Spotted when analyzing GDB backtrace. We compiled the server using
headers installed on the host, which may come from different version
of boost than the one we put in the image.

Signed-off-by: Tomasz Grabiec <tgrabiec@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>