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>

Makefile

@@ -321,7 +321,8 @@ KALLSYMS	= scripts/kallsyms
 PERL		= perl
 CHECK		= sparse
 
-CHECKFLAGS     := -D__linux__ -Dlinux -D__STDC__ -Dunix -D__unix__ -Wbitwise $(CF)
+CHECKFLAGS     := -D__linux__ -Dlinux -D__STDC__ -Dunix -D__unix__ \
+		  -Wbitwise -Wno-return-void $(CF)
 MODFLAGS	= -DMODULE
 CFLAGS_MODULE   = $(MODFLAGS)
 AFLAGS_MODULE   = $(MODFLAGS)