selftest/powerpc/ptrace/ptrace-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.

ptrace-pkey.c test has 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/20241216160257.87252-2-maddy@linux.ibm.com