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/lm4f/Makefile

@@ -44,6 +44,7 @@ OBJS += uart.o
 OBJS += vector.o
 
 OBJS += usb.o usb_control.o usb_standard.o usb_msc.o
+OBJS += usb_hid.o
 OBJS += usb_lm4f.o
 
 VPATH += ../usb:../cm3