Fix creating the list of Qt translation files under Qt5

We were apparently exploiting a bug in qmake for Qt 4. The $$join
function in Qt 5 is fixed: it returns a single string, not a list with
multiple elements.

Instead, we just iterate over the element list.

Signed-off-by: Thiago Macieira <thiago@macieira.org>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>