x86/sgx: Mark sgx_vepc_vm_ops static

Fix the following sparse warning:

  arch/x86/kernel/cpu/sgx/virt.c:95:35: warning:
    symbol 'sgx_vepc_vm_ops' was not declared. Should it be static?

This symbol is not used outside of virt.c so mark it static.

 [ bp: Massage commit message. ]

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20210412160023.193850-1-weiyongjun1@huawei.com