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)

apps/rsa.c

diff --git a/apps/rsa.c b/apps/rsa.c
index bd2f53a..807bc70 100644
--- a/apps/rsa.c
+++ b/apps/rsa.c
@@ -35,7 +35,7 @@
     OPT_NOOUT, OPT_TEXT, OPT_MODULUS, OPT_CHECK, OPT_CIPHER
 } OPTION_CHOICE;
 
-OPTIONS rsa_options[] = {
+const OPTIONS rsa_options[] = {
     {"help", OPT_HELP, '-', "Display this summary"},
     {"inform", OPT_INFORM, 'f', "Input format, one of DER NET PEM"},
     {"outform", OPT_OUTFORM, 'f', "Output format, one of DER NET PEM PVK"},