libxslt: use Copyright in LIC_FILES_CHKSUM instead of COPYING

libxslt: use Copyright in LIC_FILES_CHKSUM instead of COPYING

* COPYING is replaced by symlink to Copyright during do_configure
  (see configure.in), then we end with link to nonexistent file like this:

OE om-gta02@shr ~/shr-core $ ll tmp/deploy/licenses/libxslt/
total 40
drwxr-xr-x   2 bitbake bitbake  4096 Nov  2 00:27 ./
drwxr-xr-x 818 bitbake bitbake 32768 Nov  2 00:27 ../
lrwxrwxrwx   1 bitbake bitbake     9 Nov  2 00:27 COPYING ->  Copyright
lrwxrwxrwx   1 bitbake bitbake    52 Nov  2 00:27 generic_MIT ->  /OE/shr-core/tmp/deploy/licenses/common-licenses/MIT

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>