gnucash maint: Fix fencepost error in calculating the week_num for POSIX timezone rules.

[John Ralls]

Updated	 via  https://github.com/Gnucash/gnucash/commit/a11065b8 (commit)
	from  https://github.com/Gnucash/gnucash/commit/b51d227a (commit)



commit a11065b823f14fbc32bb1fc132304e36f486c8b0
Author: John Ralls <jralls at ceridwen.us>
Date:   Mon Mar 15 17:04:39 2021 -0700

    Fix fencepost error in calculating the week_num for POSIX timezone rules.
    
    Fixes CI failure caused by US starting DST in 2021, a year in which
    March began on a Monday.



Summary of changes:
 bindings/python/time64.i                      |  2 ++
 libgnucash/engine/gnc-timezone.cpp            |  2 +-
 libgnucash/engine/test/gtest-gnc-datetime.cpp | 35 +++++++++++++++++++++------
 3 files changed, 30 insertions(+), 9 deletions(-)