comms/birda: Fix build with default PIE.

PIE with static libraries is currently unsupported in FreeBSD, which
results in multiple errors when building with PIE. Explicitly disable
PIE for now.

PR: 253275
Submitted by: Dawid Gorecki <dgr@semihalf.com>
Approved by: mjl@luckie.org.nz (maintainer)
Differential Revision: https://reviews.freebsd.org/D28867