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/mail.c

@@ -461,7 +461,7 @@ DISABLE_WARNING_FORMAT_NONLITERAL
 void
 readmail(struct obj *otmp UNUSED)
 {
-    static const char *junk[] = {
+    static const char *const junk[] = {
         "Report bugs to <%s>.", /*** must be first entry ***/
         "Please disregard previous letter.",
         "Welcome to NetHack.",