Bug 16194: Do not consider xslt as valid theme dir in LangInstaller.pm

 Bug 16194: Do not consider xslt as valid theme dir in LangInstaller.pm

Bug 15527 add an xslt dir, LangInstaller.pm must ingnore that dir.

To test:
1) Verify the problem on current master
Update translation for any lang, will see errors

2) Apply the patch

3) Update again, no errors

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Works as expected.

Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Brendan A Gallagher <brendan@bywatersolutions.com>