print/panda: Unbreak build on FreeBSD >= 12.1

ld: error: /wrkdirs/usr/ports/print/panda/work/panda-0.5.4/.libs/libpanda.so: undefined reference to compress2

http://beefy6.nyi.freebsd.org/data/121amd64-default/529299/logs/errors/panda-0.5.4_9.log

Explicitly link with zlib.