kbuild: remove unneeded '+' marker from kselftest-merge

This line contains $(MAKE), so Make knows that it will invoke sub-make
without help of the '+' marker.
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

Makefile

@@ -1250,7 +1250,7 @@ kselftest-merge:
 	$(if $(wildcard $(objtree)/.config),, $(error No .config exists, config your kernel first!))
 	$(Q)find $(srctree)/tools/testing/selftests -name config | \
 		xargs $(srctree)/scripts/kconfig/merge_config.sh -m $(objtree)/.config
-	+$(Q)$(MAKE) -f $(srctree)/Makefile olddefconfig
+	$(Q)$(MAKE) -f $(srctree)/Makefile olddefconfig
 
 # ---------------------------------------------------------------------------
 # Devicetree files