libxsettings-client: fix COPYING file

libxsettings-client: fix COPYING file

Fixed:
* Move the code of copy COPYING file from do_configure_append() to
  do_patch[postfuncs] since we had moved license-checksum from
  do_package_qa to do_populate_lic.
* Add xsettings-client.c and xsettings-common.c to LIC_FILES_CHKSUM.
* Update comments.

Signed-off-by: Robert Yang <liezhi.yang@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>