add missing const

If you want to declare a pointer which the address pointed to is constant,
you should declare it as like `static const char *const var = "...";`.

This commit supplies missing `const` and prevents some programming
error in the future.

src/nhlua.c

@@ -53,7 +53,7 @@ static void init_u_data(lua_State *);
 static int nhl_set_package_path(lua_State *, const char *);
 static int traceback_handler(lua_State *);
 
-static const char *nhcore_call_names[NUM_NHCORE_CALLS] = {
+static const char *const nhcore_call_names[NUM_NHCORE_CALLS] = {
     "start_new_game",
     "restore_old_game",
     "moveloop_turn",