selftests, arm64: add kernel headers path for tags_test

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>