selftests/x86/protection_keys: Fix syscall NR redefinition warnings

On new enough glibc, the pkey syscalls numbers are available.  Check
first before defining them to avoid warnings like:

protection_keys.c:198:0: warning: "SYS_pkey_alloc" redefined

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bpetkov@suse.de>
Cc: Dave Hansen <dave.hansen@intel.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: stable@vger.kernel.org
Link: http://lkml.kernel.org/r/1fbef53a9e6befb7165ff855fc1a7d4788a191d6.1509794321.git.luto@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>