FROMLIST: android: binder: Change binder_shrinker to static

(from https://patchwork.kernel.org/patch/9990321/

)

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

Bug: 63926541
Change-Id: I7a13d4ddbaaf3721cddfe1d860e34c7be80dd082
Acked-by: Arve Hjønnevåg <arve@android.com>
Signed-off-by: Sherry Yang <sherryy@android.com>