Backport fixes for GCC 8.1

[tenderlove]

Hi,

This PR just backports abcminiuser/lufa@d8e0d67 and abcminiuser/lufa@a08a024 so that QMK compiles with avr-gcc 8.1.

Before this PR, I would get:

Compiling: lib/lufa/LUFA/Drivers/USB/Class/Device/AudioClassDevice.c                               In file included from lib/lufa/LUFA/Drivers/USB/Class/Device/AudioClassDevice.c:38:
lib/lufa/LUFA/Drivers/USB/Class/Device/AudioClassDevice.h:382:10: error: 'EVENT_Audio_Device_StreamStartStop' alias between functions of incompatible types 'void(USB_ClassInfo_Audio_Device_t * const)' {aka 'void(struct <anonymous> * const)'} and 'void(void)' [-Werror=attribute-alias]
     void EVENT_Audio_Device_StreamStartStop(USB_ClassInfo_Audio_Device_t* const AudioInterfaceInfo)
          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
lib/lufa/LUFA/Drivers/USB/Class/Device/AudioClassDevice.c:191:6: note: aliased declaration here
 void Audio_Device_Event_Stub(void)
      ^~~~~~~~~~~~~~~~~~~~~~~
cc1: all warnings being treated as errors
 [ERRORS]
 | 
 | 
 | 
make[1]: *** [.build/obj_planck_light_tenderlove/lib/lufa/LUFA/Drivers/USB/Class/Device/AudioClassDevice.o] Error 1
make: *** [planck/light:tenderlove] Error 1
Make finished with errors

After this PR everything seems to compile. I think this also fixes #3316

Refs: abcminiuser/lufa#130

[evantravers]

If this gets approved and merged, we should probably revert this PR: #3337

[carlf]

I merged this into my local copy and it fixed all my build problems on arch.

[skullydazed]

Thanks!