selftest/powerpc/ptrace/core-pkey: Remove duplicate macros

./powerpc/ptrace/Makefile includes flags.mk. In flags.mk,
-I$(selfdir)/powerpc/include is always included as part of
CFLAGS. So it will pick up the "pkeys.h" defined in
powerpc/include.

core-pkey.c test has couple of macros defined which
are part of "pkeys.h" header file. Remove those
duplicates and include "pkeys.h"

Reviewed-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
Signed-off-by: Madhavan Srinivasan <maddy@linux.ibm.com>
Link: https://patch.msgid.link/20241216160257.87252-1-maddy@linux.ibm.com