x86, mce: Fix compilation with !CONFIG_DEBUG_FS in mce-severity.c

Fix compilation error in arch/x86/kernel/cpu/mcheck/mce-severity.c
when CONFIG_DEBUG_FS is disabled, introduced in commit
5be9ed251f58881dfc3dd6742a81ff9ad1a7bb04.

Signed-off-by: Andi Kleen <ak@linux.intel.com>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>