ANDROID: force struct page_vma_mapped_walk to be defined in KMI

A vendor hook was recently defined that references
struct page_vma_mapped_walk, but it is only forward-declared
and therefore not fully defined in the KMI. Add inclusion of
linux/rmap.h to vender_hooks.c to add the full definition.

Bug: 233047575
Signed-off-by: Todd Kjos <tkjos@google.com>
Change-Id: I3bbaca92a70e4464e370e987ae4154de19c4fee2