selftests, arm64: fix uninitialized symbol in tags_test.c

Fix tagged_ptr not being initialized when TBI is not enabled.

Link: https://www.spinics.net/lists/linux-kselftest/msg09446.html


Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
Signed-off-by: Will Deacon <will@kernel.org>