heimdal: Do not build a redundant source file

Heimdal's lib/hdb/db3.c is only built if DB3 is enabled, i.e. #if HAVE_DB3.
FreeBSD's bdb is DB1. Therefore the entire db3.c file is #ifdef'd out.
Let's avoid building a file that results in a useless object file.

MFC after: 1 week