*: make sure `config.h` or `zebra.h` is first

*: make sure `config.h` or `zebra.h` is first

`config.h` has all the defines from autoconf, which may include things
that switch behavior of other included headers (e.g. _GNU_SOURCE
enabling prototypes for additional functions.)

So, the first include in any `.c` file must be either `config.h` (with
the appropriate guard) or `zebra.h` (which includes `config.h` first
thing.)

Signed-off-by: David Lamparter <equinox@opensourcerouting.org>