UPSTREAM: selftests, arm64: add kernel headers path for tags_test

(Upstream commit bd3841cd3bd852e7363218e71e48cb473de4fc06).

tags_test.c relies on PR_SET_TAGGED_ADDR_CTRL/PR_TAGGED_ADDR_ENABLE being
present in system headers. When this is not the case the build of this
test fails with undeclared identifier errors.

Fix by providing the path to the KSFT installed kernel headers in CFLAGS.

Reported-by: Cristian Marussi <cristian.marussi@arm.com>
Suggested-by: Cristian Marussi <cristian.marussi@arm.com>
Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
Signed-off-by: Will Deacon <will@kernel.org>
Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
Bug: 135692346
Change-Id: I60d1538e2fc391bcf74823ddcdce7c425d6bf707