Silence compiler warnings about the incf and decf functions

The code uses two functions from the ‘cl’ library but does not load
that library.  So when we compile pomodoro.el we receive warnings
about those functions if no other package has loaded ‘cl’.  Admittedly
this is an unlikely situation, as many modern packages use that
library, but it is useful to ensure ‘cl’ is loaded so that we do not
see the warnings if we are (for example) compiling pomodoro.el using
‘emacs -Q’ for testing reasons.

Signed-off-by: Eric James Michael Ritz <lobbyjones@gmail.com>
Merge pull request #19 from ejmr/require-cl

Silence Compiler Warnings About 'cl' Functions