ia64: fix build error with !COREDUMP

Fix linkage error when CONFIG_BINFMT_ELF is selected but CONFIG_COREDUMP
is not:

    ia64-linux-ld: arch/ia64/kernel/elfcore.o: in function `elf_core_write_extra_phdrs':
    elfcore.c:(.text+0x172): undefined reference to `dump_emit'
    ia64-linux-ld: arch/ia64/kernel/elfcore.o: in function `elf_core_write_extra_data':
    elfcore.c:(.text+0x2b2): undefined reference to `dump_emit'

Fixes: 1fcccbac

 ("elf coredump: replace ELF_CORE_EXTRA_* macros by functions")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Tony Luck <tony.luck@intel.com>
Cc: Fenghua Yu <fenghua.yu@intel.com>
Cc: <stable@vger.kernel.org>
Link: https://lkml.kernel.org/r/20200819064146.12529-1-krzk@kernel.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>