fix warning unused-but-set-variable 'alg_k' (no-dh and no-ec)

This patch fixes the following warning when OpenSSL is configured with
no-dh and no-ec:

./Configure no-ec no-dh linux-x86_64

...
s3_lib.c: In function 'ssl3_get_req_cert_type':
s3_lib.c:4234:19: warning: variable 'alg_k' set but not used [-Wunused-but-set-variable]
     unsigned long alg_k;

CLA: trivial
Signed-off-by: Cristian Stoica <cristian.stoica@nxp.com>

Reviewed-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/6087)