po: Fixup invalid format string

Will fix the following errors with gettext 0.22

oc.po:5301: 'msgstr' is not a valid C format string, unlike 'msgid'. Reason: In the directive number 1, the argument size specifier is invalid. oc.po:5306: 'msgstr' is not a valid C format string, unlike 'msgid'. Reason: In the directive number 1, the argument size specifier is invalid. oc.po:5312: 'msgstr' is not a valid C format string, unlike 'msgid'. Reason: In the directive number 1, the argument size specifier is invalid.

This is only found in vlc-3.x branch (not in master).