lib: make main have no args

I'm not sure when this changed, but without this patch the app does not
seem to run, and with it the blinky_delay test app works as expected.

This is also how it looks in cortex-m-rt:
https://github.com/rust-embedded/cortex-m-rt/blob/6c269f195c9a705162fd912beb952b430cc28336/src/lib.rs#L468