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/efm32/ezr32wg/Makefile

@@ -57,6 +57,7 @@ OBJS += wdog_common.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_efm32.o
 
 VPATH += ../../usb:../:../../cm3:../common

lib/efm32/hg/Makefile

@@ -44,6 +44,7 @@ OBJS += timer_common.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_dwc_common.o usb_efm32hg.o
 
 VPATH += ../../usb:../:../../cm3:../common

lib/efm32/lg/Makefile

@@ -57,6 +57,7 @@ OBJS += wdog_common.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_efm32.o
 
 VPATH += ../../usb:../:../../cm3:../common

lib/efm32/wg/Makefile

@@ -57,6 +57,7 @@ OBJS += wdog_common.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_efm32.o
 
 VPATH += ../../usb:../:../../cm3:../common