softether: add patch to fix build on uClibc toolchain without ifaddr support

This commit adds a patch to softether to make sure it doesn't use the
ifaddrs functions when they are not provided by the C library, which
can be the case with certain external uClibc toolchains.

Fixes:

  http://autobuild.buildroot.net/results/199/19992951da34a703a476dc62ea92e12927c8a02d/



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>