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>
diff --git a/Makefile b/Makefile
index 668e65c..3f95d1d 100644
--- a/Makefile
+++ b/Makefile
@@ -1250,7 +1250,7 @@
 	$(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
 
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