Add usbh_midi.c to CMakeLists.txt

[asavartsov]

#611 added Middlewares/ST/STM32_USB_Host_Library/Class/MIDI/Src/usbh_midi.c to Makefile but not to CMakeLists.txt.

Let's maybe have it because without it you can't really have USB MIDI working (even if you don't use host option) using provided CMakeLists.txt because of missing symbols during link time.