arm64: Remove unsued extern declaration init_mem_pgprot()

commit a501e324 ("arm64: Clean up the default pgprot setting")
left behind this.

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Reviewed-by: Anshuman Khandual <anshuman.khandual@arm.com>
Link: https://lore.kernel.org/r/20230720143555.26044-1-yuehaibing@huawei.com


Signed-off-by: Will Deacon <will@kernel.org>