sh: Fix sh build failure when CONFIG_SFC=m

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

Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>