s_server: fix warnings unused-but-set-variable (no-dh)

This patch fixes the following two warnings when OpenSSL is built with no-dh option:

s_server.c: In function 's_server_main':
s_server.c:1105:25: warning: variable 'no_dhe' set but not used [-Wunused-but-set-variable]
     int no_tmp_rsa = 0, no_dhe = 0, no_ecdhe = 0, nocert = 0;
                         ^
s_server.c:1101:11: warning: variable 'dhfile' set but not used [-Wunused-but-set-variable]
     char *dhfile = NULL;
           ^
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)