Bug 15395: Do not process template files that do not use i18n.inc

Bug 15395: Do not process template files that do not use i18n.inc

It should make the string extraction process a little faster

For the record, I timed some parts of the process to see why it was so
slow, and without surprises the most time-consuming task is the Perl
code analysis by PPI with an average time of 50ms by "template block" on
my machine.
Multiply that by the number of template blocks (approximatively 900),
that gives us 45 seconds just for this task.

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>