Use cl-lib macro instead of cl.el one

[syohex]

cl.el is deprecated. This also fixes the following byte-compile warnings.

org-time-budgets.el:139:41: Warning: the function ‘workweek’ is not known to
    be defined.
org-time-budgets.el:115:21: Warning: the function ‘week’ is not known to be
    defined.
org-time-budgets.el:114:21: Warning: the function ‘workday’ is not known to be
    defined.
org-time-budgets.el:113:21: Warning: the function ‘day’ is not known to be
    defined.
org-time-budgets.el:112:19: Warning: the function ‘case’ is not known to be
    defined.