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/apply.c

@@ -4046,7 +4046,7 @@ flip_through_book(struct obj *obj)
     } else if (obj->otyp == SPE_NOVEL) {
         pline("This looks like it might be interesting to read.");
     } else {
-        static const char *fadeness[] = {
+        static const char *const fadeness[] = {
             "fresh",
             "slightly faded",
             "very faded",