stubdom: recurse into tools/include in mk-headers-$(XEN_TARGET_ARCH) rule

stubdom: recurse into tools/include in mk-headers-$(XEN_TARGET_ARCH) rule

... rather than in the libxc rule.

This puts all the header dependencies in one place and will allow us
to avoid races when more libraries which need these headers are
introduced. I observed issues with the xen-foreign/tmp.size file
getting deleted in parallel with another process trying to use it.

The mini-os links are already created in the
mk-headers-$(XEN_TARGET_ARCH) target so the other places which do so
are redundant, in the case of polarssl and vtpmmgr indirectly through
their eventual dependency on newlib which in turn depends on
mk-headers-$(XEN_TARGET_ARCH).

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
Acked-by: Wei Liu <wei.liu2@citrix.com>
Cc: samuel.thibault@ens-lyon.org