selftest/powerpc/mm/pkey: fix build-break introduced by commit 00894c3

Build break was reported in the powerpc mailing list for next-20250218 with below errors

make[1]: Nothing to be done for 'all'.
BUILD_TARGET=/root/venkat/linux-next/tools/testing/selftests/powerpc/mm; mkdir -p $BUILD_TARGET; make OUTPUT=$BUILD_TARGET -k -C mm all
  CC       pkey_exec_prot
In file included from pkey_exec_prot.c:18:
/root/venkat/linux-next/tools/testing/selftests/powerpc/include/pkeys.h: In function ‘pkeys_unsupported’:
/root/venkat/linux-next/tools/testing/selftests/powerpc/include/pkeys.h:96:34: error: ‘PKEY_UNRESTRICTED’ undeclared (first use in this function)
   96 |         pkey = sys_pkey_alloc(0, PKEY_UNRESTRICTED);
      |                                  ^~~~~~~~~~~~~~~~~

https://lore.kernel.org/all/20250113170619.484698-2-yury.khrustalev@arm.com/ patchset
has been queued to arm64/for-next/pkey_unrestricted which is causing a build break
in the selftest/powerpc builds.

Commit 6d61527 ("mm/pkey: Add PKEY_UNRESTRICTED macro") added a macro
PKEY_UNRESTRICTED to handle implicit literal value of 0x0 (which is "unrestricted").
Add the same to selftest/powerpc/pkeys.h to fix the reported build break.

Fixes: 00894c3 ("selftests/powerpc: Use PKEY_UNRESTRICTED macro")
Reported-by: Venkat Rao Bagalkote <venkat88@linux.ibm.com>
Closes: https://lore.kernel.org/lkml/3267ea6e-5a1a-4752-96ef-8351c912d386@linux.ibm.com/T/
Tested-by: Venkat Rao Bagalkote <venkat88@linux.ibm.com>
Signed-off-by: Madhavan Srinivasan <maddy@linux.ibm.com>
Link: https://lore.kernel.org/r/20250311084129.39308-1-maddy@linux.ibm.com
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

tools/testing/selftests/powerpc/include/pkeys.h

@@ -24,6 +24,9 @@
 #undef PKEY_DISABLE_EXECUTE
 #define PKEY_DISABLE_EXECUTE	0x4
 
+#undef PKEY_UNRESTRICTED
+#define PKEY_UNRESTRICTED	0x0
+
 /* Older versions of libc do not define this */
 #ifndef SEGV_PKUERR
 #define SEGV_PKUERR	4