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POTFILES.in should not contain src/python/__init__.py file as it
is not present in the committed tree. It has its respective .in file
which is present instead.

This commit further ensures po/POTFILES.in generator avoids such
files that Git ignores (using git-check-ignore during find).
Signed-off-by: Valters Jansons <valter.jansons@gmail.com>
Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>