ignore: Add *.dtd.embed

Although autogen.sh has a --disable-dtd-validation option, it is
on by default, so it seems convenient to add the generated symlink
to .gitignore.

Signed-off-by: Yong Bakos <ybakos@humanoriented.com>
Reviewed-by: Bryce Harrington <bryce@osg.samsung.com>