android: binder: Change binder_shrinker to static

binder_shrinker struct is not used anywhere outside of
binder_alloc.c and should be static.

Acked-by: Arve Hjønnevåg <arve@android.com>
Signed-off-by: Sherry Yang <sherryy@android.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>