sh: x3 - fix setup_bootmem_node() compile error with shx3_defconfig

This makes sure the function prototype for setup_bootmem_node() gets
included. The file setup-shx3.c does not compile otherwise for
CONFIG_NUMA=n.

Signed-off-by: Magnus Damm <damm@igel.co.jp>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>