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.

Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>