pceplib: add missing endian.h and config.h

pceplib: add missing endian.h and config.h

endian.h supplies be*toh() and htobe*() functions.  This fixes the build
on musl libc.  On other systems it seems endian.h comes in transitively
from some other header.

(Also, all .c files should have config.h or zebra.h as the first
include, even if it works without that it's b0rked and only works due to
luck.)

Tested-by: Lucian Cristian <lucian.cristian@gmail.com>
Signed-off-by: David Lamparter <equinox@diac24.net>