follow-up: cross compile for msdos

sys/msdos/msdos.c

@@ -449,7 +449,7 @@ filesize_nh(char *file)
  * Chdrive() changes the default drive.
  */
 void
-chdrive(char *str)
+chdrive(const char *str)
 {
 #define SELECTDISK 0x0E
     char *ptr;