fs/proc/vmcore.c: fix warning when CONFIG_MMU=n

>> fs/proc/vmcore.c:424:19: warning: 'mmap_vmcore_fault' defined but not used [-Wunused-function]
     424 | static vm_fault_t mmap_vmcore_fault(struct vm_fault *vmf)

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202411140156.2o0nS4fl-lkp@intel.com/


Cc: Qi Xi <xiqi2@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>