package/libmicrohttpd: security bump version to 0.9.55

For details refer to release notes:
http://lists.gnu.org/archive/html/info-gnu/2017-05/msg00014.html



Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>