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/artifact.c

@@ -1883,7 +1883,7 @@ glow_color(int arti_indx)
 }
 
 /* glow verb; [0] holds the value used when blind */
-static const char *glow_verbs[] = {
+static const char *const glow_verbs[] = {
     "quiver", "flicker", "glimmer", "gleam"
 };