kbuild: disable sparse warning "returning void-valued expression"

The sparse warning -Wreturn-void ("returning void-valued expression")
is off by default, but it is enabled with -Wall, so add
-Wno-return-void to CHECKFLAGS to disable it.

Signed-off-by: Hannes Eder <hannes@hanneseder.net>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>