USB: linux/usb/Kbuild needs only "header-y", not "unifdef-y"

Given that none of the referenced header files test the proprocessor
conditional __KERNEL__, there's no point "unifdef"fing them.
Signed-off-by: NRobert P. J. Day <rpjday@crashcourse.ca>
Signed-off-by: NGreg Kroah-Hartman <gregkh@suse.de>

include/linux/usb/Kbuild

-unifdef-y += audio.h
-unifdef-y += cdc.h
-unifdef-y += ch9.h
-unifdef-y += gadgetfs.h
-unifdef-y += midi.h
+header-y += audio.h
+header-y += cdc.h
+header-y += ch9.h
+header-y += gadgetfs.h
+header-y += midi.h
 unifdef-y += g_printer.h