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/shk.c

@@ -83,7 +83,7 @@ static const char *cad(boolean);
                     obj->quan <= bp->bquan
  */
 
-static const char *angrytexts[] = { "quite upset", "ticked off", "furious" };
+static const char *const angrytexts[] = { "quite upset", "ticked off", "furious" };
 
 /*
  *  Transfer money from inventory to monster when paying