kbuild: Fix 'make -jN' warning.

make doesn't recognize itself that it does a recursive call, so we have
to use a leading '+' to let it know.

(reported by Steven Cole)

Makefile

@@ -352,7 +352,7 @@ endef
 #	set -e makes the rule exit immediately on error
 
 define rule_vmlinux__
-	set -e;								\
+	+set -e;							\
 	$(if $(filter .tmp_kallsyms%,$^),,				\
 	  echo '  GEN     .version';					\
 	  . $(srctree)/scripts/mkversion > .tmp_version;		\