define _XOPEN_SOURCE to 700 for strptime(3)

Defining _OPEN_SOURCE alone is not enough, it must be set to 700 to identify
POSIX 2008 on at least OpenBSD to prevent the following warning:

../gnome-todo-3.28.1/plugins/todoist/gtd-provider-todoist.c:293:8: warning: implicit declaration of function 'strptime' is invalid in C99 [-Wimplicit-function-declaration]
  if (!strptime (due_date, "%a %d %b %Y %T %z", &due_dt))
1 job from strptime in 18 minutes and 19 seconds (queued for 3 seconds)
latest
Status Job ID Name Coverage
  Flatpak
passed #52465
flatpak:master

00:18:19