bcusdk: make it build with the musl C library

bcusdk: make it build with the musl C library

This commit makes bcusdk build with the musl C library by:

 * Adding a dependency on argp-standalone

 * Adding a patch to fix the incorrect usage of <sys/cdefs.h>

 * Adding a patch to fix a missing header include for fd_set

Both patches have been submitted upstream,
https://sourceforge.net/p/bcusdk/patches/3/.

Fixes:

  http://autobuild.buildroot.net/results/46054f28d3fde173dd8f880fa8ac2784c8e1750f/

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