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/ball.c

@@ -887,7 +887,7 @@ drop_ball(xchar x, xchar y)
     }
 
     if (x != u.ux || y != u.uy) {
-        static const char *pullmsg = "The ball pulls you out of the %s!";
+        static const char *const pullmsg = "The ball pulls you out of the %s!";
         struct trap *t;
         long side;