ANDROID: force struct selinux_state to be defined in KMI

struct selinux_state is defined in security/selinux/include/security.h,
however libabigail is not finding its definition based on the
instantiation of the hooks, so force it to be defined by defining a
dummy exported symbol. Since blk_mq_alloc_data is defined in a
subsystem-private header, create a new vendor_hooks.c file in
security/selinux to define the dummy symbol.

Bug: 233047575
Signed-off-by: Todd Kjos <tkjos@google.com>
Change-Id: Ia505c76db2eed339b3815073f847b500535cc954