Suppress -Wcast-qual warnings in bsearch

The first cast to (void *) is redundant but should be (const void *)
anyway, because that's the type of the lvalue being assigned to.

The second cast is necessary and intentionally not const-correct, so
tell the compiler not to warn about it.

Reviewed-by: Florian Weimer <fweimer@redhat.com>
(cherry picked from commit a725ff1d)