MIPS: Make setup_elfcorehdr and setup_elfcorehdr_size static

This addresses the following sparse warning:

arch/mips/kernel/setup.c:446:33: warning: symbol 'setup_elfcorehdr_size'
was not declared. Should it be static?

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Jason Yan <yanaijie@huawei.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>