libkafs5 needs a header from libkrb5, it includes this from
${.OBJDIR}/mumble, this used to work fine as long as you happen to have
a krb_err.h in your base system, this doesn't work for bootstrapping or
using a cross-compiler with a different sysroot. This is just a
best-effort bandaid, sufficient parallelism can still break it.
Fix a SRCS override that dropped krb5_err.h.
Discussed with: stas
3abde52372