[make] add -Werror=return-type
[make] add -Werror=return-type

Why gcc just warns about no return statement in a function that is supposed
to return something, I dunno. Fix that glitch.

ErikCorryGoogle/lk,travisg/lk,minglun-tsai/lk,srodrig1/lk,srodrig1/lk,skabet/lk,hollanderic/lkstuff,skabet/lk,sndnvaps/lk,ErikCorryGoogle/lk,sndnvaps/lk,skabet/lk,skabet/lk,minglun-tsai/lk,hollanderic/lkstuff,hollanderic/lkstuff,travisg/lk,ErikCorryGoogle/lk,minglun-tsai/lk,minglun-tsai/lk,skabet/lk,littlekernel/lk,nvll/lk,travisg/lk,srodrig1/lk,nvll/lk,srodrig1/lk,ErikCorryGoogle/lk,sndnvaps/lk,srodrig1/lk,nvll/lk,littlekernel/lk,sndnvaps/lk,travisg/lk,nvll/lk,littlekernel/lk,hollanderic/lkstuff,littlekernel/lk,sndnvaps/lk,nvll/lk