src/readelf: use qsort instead of qsort_r.

src/readelf: use qsort instead of qsort_r.

This program is single threaded, so using qsort with a global variable
isn't a danger. The interface for qsort_r isn't standardized (and
diverges between glibc and FreeBSD, for example), which makes usage of
qsort, where possible, preferrable.

Signed-off-by: Érico Rolim <erico.erc@gmail.com>