SDL has now been initialized

I finally got a bit of time to get mod_time working.
The functions seem to work fine, so now the three functions it contains (ftime, get_timer and time) seem to work fine.

The nice thing is that this means that SDL is being initialized at the program startup. This is important as SDL is the main library behind Bennu, so it's always nice to see it not crashing.

I also added the test case for this library (examples/05_mod_time.prg, 04_mod_math.prg is missing, sorry for that) in case you wanna give it a try.

More updates to come.