x86/boot/KASLR: Make local variable mem_limit static

Fix the following sparse warning:

arch/x86/boot/compressed/kaslr.c:102:20: warning: symbol 'mem_limit' was not declared. Should it be static?

Signed-off-by: zhong jiang <zhongjiang@huawei.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: <gregkh@linuxfoundation.org>
Link: https://lkml.kernel.org/r/1532958273-47725-1-git-send-email-zhongjiang@huawei.com