selftests/mm: enable mrelease_test for arm64

selftests/mm: enable mrelease_test for arm64

mrelease_test defaults to defining __NR_pidfd_open and
__NR_process_mrelease syscall numbers to -1, if they are not defined
anywhere else, and the suite would then be marked as skipped as a result.

arm64 (at least the stock debian toolchain that I'm using) requires
including <sys/syscall.h> to pull in the defines for these syscalls.  So
let's add this header.  With this in place, the test is passing on arm64.

Link: https://lkml.kernel.org/r/20230724082522.1202616-4-ryan.roberts@arm.com
Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Cc: Florent Revest <revest@chromium.org>
Cc: Jérôme Glisse <jglisse@redhat.com>
Cc: John Hubbard <jhubbard@nvidia.com>
Cc: Mark Brown <broonie@kernel.org>
Cc: Peter Xu <peterx@redhat.com>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>