coredump: Move dump_emit_page() to kill unused warning

If CONFIG_ELF_CORE is not set:

    fs/coredump.c:835:12: error: ‘dump_emit_page’ defined but not used [-Werror=unused-function]
      835 | static int dump_emit_page(struct coredump_params *cprm, struct page *page)
          |            ^~~~~~~~~~~~~~

Fix this by moving dump_emit_page() inside the existing section
protected by #ifdef CONFIG_ELF_CORE.

Fixes: 06bbaa6d ("[coredump] don't use __kernel_write() on kmap_local_page()")
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>