package/libsemanage: disable on musl

Suggested by Thomas:
http://article.gmane.org/gmane.comp.lib.uclibc.buildroot/149138


"getpwent_r() is a glibc-specific extension, so it will most likely not
 be implemented by musl."

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>