gettext.php breaks on poedit plural po files

[nbachiyski]

poEdit doesn't put a semi-colon at the end of plural headers (even if explicitly told so). However the standard requires them and out gettext implementation blows with parse error (it uses eval) if a semi-colon is missing.

So I added one ;)

P.S. There is no problem to have more than one semi-colon at the end of a statement, so I skipped the sanity check if there actually *is* a semi-colon at the end of the header.

(In [4736]) Fix plural form parsing. Props nbachiyski. fixes #3577