fix portability of Piece.xs

On HP-UX 11.31 with HP's ANSI compiler:

"Piece.xs", line 966: error #3031-D: an entity with internal linkage cannot
be referenced within an inline function with external linkage
        return _strptime( buf, fmt, tm, &got_GMT);
               ^

See perl5-porters@perl.org

Message-ID: <20140102102027.473cc657@pc09.procura.nl>
prep next release