cdefs: Remove func define

We require an ANSI-C compiler to build the base system. It's required
that func work. Remove this define since the only known problem
compilers are ancient history (gcc 2.6 from 1994, almost pre-dating the
project). 3rd party code that used this define will now need to provide
it via some other means when using non-ansi-c compilers.

PR: 275221 (exp-run)
Sponsored by: Netflix