libsolv: Enable correct qsort_r on musl

Newer musl now has this function implemented and signatures are same as
glibc, current codepaths forces it on BSD path which is wrong for musl
and results in random segfaults in apps using libsolv e.g. opkg

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>