domain: Allow stat on symlinks in vendor

Addresses:
denied { getattr } for pid=155 comm="keystore" path="/vendor"
dev="mmcblk0p6" ino=1527 scontext=u:r:keystore:s0
tcontext=u:object_r:system_file:s0 tclass=lnk_file

On devices without an actual vendor image, /vendor is a symlink to
/system/vendor. When loading a library from this symlinked vendor,
the linker uses resolve_paths() resulting in an lstat(). This
generates an selinux denial. Allow this lstat() so that paths can
be resolved on devices without a real vendor image.

Bug: 35946056
Test: sailfish builds
Change-Id: Ifae11bc7039047e2ac2b7eb4fbcce8ac4580799f