scripts/spdxcheck.py: handle license identifiers in XML comments

Commit cc9539e7

 ("media: docs: use the new SPDX header for GFDL-1.1 on
*.svg files") adds SPDX-License-Identifiers enclosed in XML comments,
i.e., <!-- ... -->, for svg files.

Unfortunately, ./scripts/spdxcheck.py does not handle
SPDX-License-Identifiers in XML comments, so it simply fails on checking
these files with 'Invalid License ID: --'.

Strip the XML comment ending simply by copying how it was done for comments
in C. With that, ./scripts/spdxcheck.py handles the svg files properly.

Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>