gdb/copyright.py: Also update sources in "gdbserver" and "gdbsupport"

This commit adjusts GDB's copyright.py script, following two past changes:
  - gdb/gdbserver/ being move to the toplevel directory;
  - gdb/common/ being renamed to gdbsupport/.

gdb/ChangeLog:

        * copyright.py (get_update_list): Add "gdbserver" and "gdbsupport"
        to the list of directories to update.