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/engine.c

diff --git a/apps/engine.c b/apps/engine.c
index ffd3137..da1d918 100644
--- a/apps/engine.c
+++ b/apps/engine.c
@@ -26,7 +26,7 @@
     OPT_V = 100, OPT_VV, OPT_VVV, OPT_VVVV
 } OPTION_CHOICE;
 
-OPTIONS engine_options[] = {
+const OPTIONS engine_options[] = {
     {OPT_HELP_STR, 1, '-', "Usage: %s [options] engine...\n"},
     {OPT_HELP_STR, 1, '-',
         "  engine... Engines to load\n"},