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/stm32/f4/Makefile

@@ -65,6 +65,7 @@ OBJS += timer_common_all.o timer_common_f0234.o timer_common_f24.o
 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_dwc_common.o usb_f107.o usb_f207.o
 
 OBJS += mac.o phy.o mac_stm32fxx7.o phy_ksz80x1.o