include/linux/mm_types.h: ifdef struct vm_area_struct::swap_readahead_info

The field is only used in swap code.

Link: http://lkml.kernel.org/r/20190503190500.GA30589@avx2
Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>