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/lm4f/Makefile

@@ -45,6 +45,7 @@ OBJS += vector.o
 
 OBJS += usb.o usb_control.o usb_standard.o usb_msc.o
 OBJS += usb_hid.o
+OBJS += usb_audio.o usb_cdc.o usb_midi.o
 OBJS += usb_lm4f.o
 
 VPATH += ../usb:../cm3