libxmlrpc: bump to 1.25.30 and add hash file

This commit bumps libxmlrpc to the latest available stable version,
and adds a hash file.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>