time::HiRes: don't truncate nanosec utime

When passed a floating point atime/mtime value, T::HR::utime()
was converting it into two longs: secs and nsec. But the nanosec value
was calculated using a final NV to long cast, which truncates any
fractional part rather than rounding to nearest. Use a 0.5 addition to
force rounding.

This was manifesting as a test in lib/File/Copy.t failing to preserve
the same mtime after a couple of round trips with utime() and stat().

In particular, the test was attempting to set an mtime to the literal
floating-point value

    1000000000.12345

This value can't be represented exactly as an NV, so was actually
(under -Dquadmath)

1000000000.1234499999999999999999999568211720247320

which was (using truncation) being converted into the two sec/nsec
longs:

    1000000000, 123449999

After this commit, it instead correctly gets converted to

    1000000000, 123450000

0 files changed, 0 insertions, 0 deletions