kerberos5: Migrate from COMPAT_32BIT to generic COMPAT_LIBCOMPAT

See commit 8fad2cda93c7 ("bsd.compat.mk: Provide new CPP and sub-make
variables") for the context behind this change.

Reviewed by: emaste, imp, brooks, jhb
Differential Revision: https://reviews.freebsd.org/D40926