package/systemd: explicitly --enable-microhttpd

Signed-off-by: Gabe Evans <gabe@hashrabbit.co>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
[Thomas: fix commit title.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>