Fix 'make distcheck'

Fix a bug introduced in 6aa50cc4c93fec243cba9cd37c86090496af3feb
Fix 'make distcheck'

'make distcheck' has been broken again in
c74ba603517380bae59f1ff68a5f100c2fb1c960
Remove a duplicated line

INTLTOOL_XML_RULE is used twice in Makefile.am, which results in having
two same lines in generated Makefile.
Add a missing file to POTFILES.in