stubdom: modify ioemu linkfarm only if necessary

stubdom: modify ioemu linkfarm only if necessary

Several stubdom libraries are being rebuilt each time a top level make
is called as they depend on stubdom/ioemu/linkfarm.stamp which is
depending on tools/qemu-xen-traditional-dir. Unfortunately this
directory is modified by each "make tools" call.

This can be avoided by writing stubdom/ioemu/linkfarm.stamp only if
a source file beneath tools/qemu-xen-traditional-dir has been added
or removed.

Signed-off-by: Juergen Gross <jgross@suse.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>
Reviewed-by: Samuel Thibault <samuel.thibault@ens-lyon.org>