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/lock.c

@@ -37,7 +37,7 @@ static const char *
 lock_action(void)
 {
     /* "unlocking"+2 == "locking" */
-    static const char *actions[] = {
+    static const char *const actions[] = {
         "unlocking the door",   /* [0] */
         "unlocking the chest",  /* [1] */
         "unlocking the box",    /* [2] */