linux/elfcore.h: hide kernel functions

The declarations for elf_core_extra_phdrs() et al got added on the
wrong side of #ifdef __KERNEL__ in linux/elfcore.h so they leak into
the user header copy and we get a warning at build time about it.

Signed-off-by: Roland McGrath <roland@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>