include hardware/clocks.h for set_sys_clock_khz

[jonathanschilling]

Hi, thanks for this very nice setup!
I encoutered an issue when building the mac.uf2 file on Arch Linux, related to set_sys_clock_khz not being found.
The fix was to also include #include "hardware/clocks.h" in mac.c.
Would you mind adding that include statement (I can also open a PR if needed)?

[guruthree]

Would you be able to check what version you have checked out/download when you compiled? This should already have been changed in 6bb797e.