avformat/hlsenc: don't use %s for strftime on msvc

MSVC doesn't support the %s time format, and instead of returning an
error the invalid parameter handler is invoked which (by default)
terminates the process.

Reviewed-by：Steven Liu <lq@chinaffmpeg.org>
Signed-off-by: Hendrik Leppkes <h.leppkes@gmail.com>
