add usb audio, cdc, and midi stub files for doxy

- add stub files to relevant makefiles
- include <stdint.h> in respective headers to fix compilation

lib/stm32/f4/Makefile

@@ -66,6 +66,7 @@ OBJS += usart_common_all.o usart_common_f124.o
 
 OBJS += usb.o usb_standard.o usb_control.o usb_msc.o
 OBJS += usb_hid.o
+OBJS += usb_audio.o usb_cdc.o usb_midi.o
 OBJS += usb_dwc_common.o usb_f107.o usb_f207.o
 
 OBJS += mac.o phy.o mac_stm32fxx7.o phy_ksz80x1.o