selftests/mm: Define PKEY_UNRESTRICTED for pkey_sighandler_tests

selftests/mm: Define PKEY_UNRESTRICTED for pkey_sighandler_tests

Commit 6e182dc9f268 ("selftests/mm: Use generic pkey register
manipulation") makes use of PKEY_UNRESTRICTED in
pkey_sighandler_tests. The macro has been proposed for addition to
uapi headers [1], but the patch hasn't landed yet.

Define PKEY_UNRESTRICTED in pkey-helpers.h for the time being to fix
the build.

[1] https://lore.kernel.org/all/20241028090715.509527-2-yury.khrustalev@arm.com/

Fixes: 6e182dc9f268 ("selftests/mm: Use generic pkey register manipulation")
Reported-by: Aishwarya TCV <aishwarya.tcv@arm.com>
Signed-off-by: Kevin Brodsky <kevin.brodsky@arm.com>
Link: https://lore.kernel.org/r/20241107131640.650703-1-kevin.brodsky@arm.com
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>