Fix build with po4a 0.56 via consistent "\n" use.

Errors look like so:

PERL5LIB=.. ./po2wiki underlay.setup
Invalid po file
/var/folders/y5/k8bh4xsj75g4j1n631f2gqz80000gp/T/ikiwiki-po-filter-in.OPXJxZ0juW:
/var/folders/y5/k8bh4xsj75g4j1n631f2gqz80000gp/T/ikiwiki-po-filter-in.OPXJxZ0juW:293:
'msgid' and 'msgstr' entries do not both end with '\n'
/var/folders/y5/k8bh4xsj75g4j1n631f2gqz80000gp/T/ikiwiki-po-filter-in.OPXJxZ0juW:589:
'msgid' and 'msgstr' entries do not both end with '\n'
msgfmt: found 2 fatal errors

make[1]: *** [po2wiki_stamp] Error 255