package/setools: drop path prefix from man install

package/setools: drop path prefix from man install

For this man file install scenario, joining the sys.prefix makes the
path absolute (this was previously working by accident).  It was
found when e94280e5a537b "package/pkg-python: use proper --prefix
and --root values" was merged.

Fixes:
http://autobuild.buildroot.net/results/e214e1a539cdac07028fb58c3822f89886d86f1f

Upstream:
https://github.com/SELinuxProject/setools/pull/13/commits/bc36cba1393120e65bfe4d0f642fd8d38010c0f7

Cc: Angelo Compagnucci <angelo.compagnucci@gmail.com>
Cc: Arnout Vandecappelle <arnout@mind.be>
Signed-off-by: Matthew Weber <matthew.weber@rockwellcollins.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>