kbuild: silence mystery message

During last phase of the build the following message were displayed:
/bin/sh: +@: command not found

This message appears due to slightly changed semantics
of cmd and if_changed_rule.
The easy fix was to insert a dummy command first in rule_ksym_ld.
The alternative was to redo part of this processing in the top-level
Makefile - a volatile area that I try to avoid.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
---

Makefile

@@ -688,8 +688,10 @@ endef
 
 # Update vmlinux version before link
 # Use + in front of this rule to silent warning about make -j1
+# First command is ':' to allow us to use + in front of this rule
 cmd_ksym_ld = $(cmd_vmlinux__)
 define rule_ksym_ld
+	: 
 	+$(call cmd,vmlinux_version)
 	$(call cmd,vmlinux__)
 	$(Q)echo 'cmd_$@ := $(cmd_vmlinux__)' > $(@D)/.$(@F).cmd