python-libconfig: disable on musl

The python-libconfig package fails to build with musl with very weird
errors coming all the way from Boost Python, which nobody ever
bothered to fix. It's time to disable this package on musl to avoid
the repetitive build failures.

Fixes:

  http://autobuild.buildroot.net/results/f0f6cdc8c38c024772615d5e677b0f4ad63ef7ec



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