tools: fix build dependency upon generated header(s)

tools: fix build dependency upon generated header(s)

Commit fd35f32b4b ("tools/x86emul: Use struct cpuid_policy in the
userspace test harnesses") didn't account for the dependencies of
cpuid-autogen.h to potentially change between incremental builds.
Putting the make invocation to produce the header together with the
directory tree creation therefore does not work. Introduce a separate
goal.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>
Release-acked-by: Juergen Gross <jgross@suse.com>