because long is 32bits on Windows and 64bits on Linux.

Reasoning:
(a) all the users of Fraction used to be 32bit in the past
(b) this makes the Linux code behave the same as the Windows code
(c) changing it to 64bits would be dangerous because then call sites
could see silent truncation.

Change-Id: I2a36200623a3cf2e7d62ccad030a20614e1798fb
Reviewed-on: https://gerrit.libreoffice.org/42200Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>