package/libxmlrpc: fix shared libraries build for uClibc

libxmlrpc is able to build shared libraries only for (e)glibc system
since the regexp in config.mk.in is based on "linux-gnu".

Change this regexp to match "linux-uclibc".

Signed-off-by: Romain Naour <romain.naour@openwide.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>