devel/libevent2: update to 2.1.10

Changes: https://github.com/libevent/libevent/releases/tag/release-2.1.10-stable
ABI: https://abi-laboratory.pro/tracker/timeline/libevent/
PR: 238127
Reported by: GitHub (watch releases)
Tested by: pkubaj (powerpc64)
Approved by: maintainer timeout (2 weeks)
MFH: 2019Q2 (maybe security)