[IA64] Fix ia64 build failure when CONFIG_SFC=m

CONFIG_SFC=m uses topology_core_siblings() which, for ia64, expects
cpu_core_map to be exported.  It is not.  This patch exports the needed
symbol.

Maintainers note: This really looks like the wrong thing to do ... it
would be much better for the kernel to export an API to provide
drivers like this with data they need (which in the case of this
driver seems to be an estimate of the effective parallelism available
on the platform).  But x86 has exported this forever ... so go with
the flow until such an API is defined.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: Matthew Wilcox <willy@linux.intel.com>
Signed-off-by: Robin Holt <holt@sgi.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>