deskutils/gsimplecal: Fix build with Clang 6

MainWindow.cpp:157:33: error: C-style cast from 'nullptr_t' to 'GtkAccelFlags' is not allowed

(GtkAccelFlags)NULL, closure);
^~~~~~~~~~~~~~~~~~~

http://beefy12.nyi.freebsd.org/data/head-amd64-default/p479076_s338486/logs/errors/gsimplecal-2.1.log

PR: 230883
Submitted by: tobik
Approved by: lme (maintainer timeout, 2 weeks)