OpenSSL-II style for emacs: don't indent because of extern block

We don't want an indentation step inside a 'extern "C" {' .. '}'
block.  Apparently, cc-mode has a c-offsets-alist keyword to allow
exactly this.

Reviewed-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Matthias St. Pierre <Matthias.St.Pierre@ncp-e.com>
(Merged from https://github.com/openssl/openssl/pull/6557)