build: fix make warning if there is no cppcheck

build: fix make warning if there is no cppcheck

If cppcheck is not present, the following warning appears during build:
which: no cppcheck in ([...])
/bin/sh: cppcheck: command not found

Fix the problem by using shell code inside the cppcheck-version rule to
also prevent unneeded call of which when something else than cppcheck is
built.

Reported-by: Julien Grall <julien@xen.org>
Signed-off-by: Bertrand Marquis <bertrand.marquis@arm.com>
Acked-by: Jan Beulich <jbeulich@suse.com>