Use portable format specifiers: Case %ld - plain - sprintf.

Fix uses of plain "%ld" within sprintf():
- Replace "%ld" with "%" PRId64.
- Cast corresponding argument to (int64_t).

src/message.c

@@ -1064,7 +1064,7 @@ void msg_outnum(long n)
 {
   char_u buf[20];
 
-  sprintf((char *)buf, "%ld", n);
+  sprintf((char *)buf, "%" PRId64, (int64_t)n);
   msg_puts(buf);
 }