Constify command options

Reviewed-by: Kurt Roeckx <kurt@openssl.org>
Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/1694)
diff --git a/apps/s_server.c b/apps/s_server.c
index 2051459..eaecb7e 100644
--- a/apps/s_server.c
+++ b/apps/s_server.c
@@ -679,7 +679,7 @@
     OPT_X_ENUM
 } OPTION_CHOICE;
 
-OPTIONS s_server_options[] = {
+const OPTIONS s_server_options[] = {
     {"help", OPT_HELP, '-', "Display this summary"},
     {"port", OPT_PORT, 'p',
      "TCP/IP port to listen on for connections (default is " PORT ")"},