databases/pgbouncer: Update to 1.12.0

• update to 1.12.0
• add USE=pkgconfig for libevent

Changes: https://github.com/pgbouncer/pgbouncer/releases/tag/pgbouncer_1_12_0

PR: 241331
Submitted by: Loïc Bartoletti <lbartoletti@tuxfamily.org>
Approved by: m.tsatsenko@gmail.com (maintainer), araujo (mentor)
Differential Revision: https://reviews.freebsd.org/D22097