Requires: libevent >= 2.0.21 to avoid crashes on 3.5 kernels with seccomp-filter

see the devel-en thread:
http://lists.pld-linux.org/mailman/pipermail/pld-devel-en/2013-February/023413.html

hardcore testing fame goes to arekm

Tested-By: arekm
Tested-By: baggins
Tested-By: glen