package/ustr: disable on musl

libsemanage is the only package depending on ustr, both packages do not
build using a musl-based toolchain, suggested by Thomas:
http://article.gmane.org/gmane.comp.lib.uclibc.buildroot/149138



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