kbuild: do not pass -r to genksyms when *.symref does not exist

There is no need to pass '-r /dev/null', which is no-op.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nicolas Schier <nicolas@fjasle.eu>

Author: masahir0y

scripts/Makefile.build

@@ -110,7 +110,7 @@ $(obj)/%.i: $(obj)/%.c FORCE
 genksyms = $(objtree)/scripts/genksyms/genksyms		\
 	$(if $(1), -T $(2))			\
 	$(if $(KBUILD_PRESERVE), -p)		\
-	-r $(or $(wildcard $(2:.symtypes=.symref)), /dev/null)
+	$(addprefix -r , $(wildcard $(2:.symtypes=.symref)))
 
 # These mirror gensymtypes_S and co below, keep them in synch.
 cmd_gensymtypes_c = $(CPP) -D__GENKSYMS__ $(c_flags) $< | $(genksyms)