sysemu/numa.h includes hw/boards.h just for the CPUArchId typedef, at
the cost of pulling in more than two dozen extra headers indirectly.

I could move the typedef from hw/boards.h to qemu/typedefs.h.  But
it's used in just two headers: boards.h and numa.h.

I could move it to another header both its users include.
exec/cpu-common.h seems to be the least bad fit.

But I'm keeping this simple & stupid: declare the struct tag in
numa.h.

Cc: Eduardo Habkost <ehabkost@redhat.com>
Cc: Marcel Apfelbaum <marcel.apfelbaum@gmail.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Message-Id: <20190812052359.30071-24-armbru@redhat.com>