usb: hid: add a stub file for doxygen

Unlike with the doxygen source generation, we can't autoguess which of
the class files are eligible automatically.  Instead, make a stub hid
file, (which we can now start adding to, if desired) and include it in
all builds that include other class stubs.

lib/efm32/ezr32wg/Makefile

@@ -56,6 +56,7 @@ OBJS += usart_common.o
 OBJS += wdog_common.o
 
 OBJS += usb.o usb_control.o usb_standard.o usb_msc.o
+OBJS += usb_hid.o
 OBJS += usb_efm32.o
 
 VPATH += ../../usb:../:../../cm3:../common

lib/efm32/hg/Makefile

@@ -43,6 +43,7 @@ OBJS += gpio_common.o
 OBJS += timer_common.o
 
 OBJS += usb.o usb_control.o usb_standard.o usb_msc.o
+OBJS += usb_hid.o
 OBJS += usb_dwc_common.o usb_efm32hg.o
 
 VPATH += ../../usb:../:../../cm3:../common

lib/efm32/lg/Makefile

@@ -56,6 +56,7 @@ OBJS += usart_common.o
 OBJS += wdog_common.o
 
 OBJS += usb.o usb_control.o usb_standard.o usb_msc.o
+OBJS += usb_hid.o
 OBJS += usb_efm32.o
 
 VPATH += ../../usb:../:../../cm3:../common

lib/efm32/wg/Makefile

@@ -56,6 +56,7 @@ OBJS += usart_common.o
 OBJS += wdog_common.o
 
 OBJS += usb.o usb_control.o usb_standard.o usb_msc.o
+OBJS += usb_hid.o
 OBJS += usb_efm32.o
 
 VPATH += ../../usb:../:../../cm3:../common