fs/proc/vmcore.c: hide vmcoredd_mmap_dumps() for nommu builds

Without CONFIG_MMU, we get a build warning:

  fs/proc/vmcore.c:228:12: error: 'vmcoredd_mmap_dumps' defined but not used [-Werror=unused-function]
   static int vmcoredd_mmap_dumps(struct vm_area_struct *vma, unsigned long dst,

The function is only referenced from an #ifdef'ed caller, so
this uses the same #ifdef around it.

Link: http://lkml.kernel.org/r/20180525213526.2117790-1-arnd@arndb.de
Fixes: 7efe48df

 ("vmcore: append device dumps to vmcore as elf notes")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: Ganesh Goudar <ganeshgr@chelsio.com>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Rahul Lakkireddy <rahul.lakkireddy@chelsio.com>
Cc: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>