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>
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