Enable USB for L1

This was tested using the example code in https://github.com/karlp/libopencm3-examples/tree/very_basic_l1_usb_demo/examples/stm32/l1/dactastic/usb_cdcacm (which is for a custom board and probably shouldn't be committed)

It has _not_ been tested on an F103 board to double check that the rcc include is correct, but if that wasn't ok, we'd have much bigger problems in other places :)