x86/mpx: Re-add MPX to selftests Makefile

x86/mpx: Re-add MPX to selftests Makefile

Ingo pointed out that the MPX tests were no longer in the selftests
Makefile.  It appears that I shot myself in the foot on this one
and accidentally removed them when I added the pkeys tests, probably
from bungling a merge conflict.

Reported-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Fixes: a0091bbe41a7 ("x86/pkeys: Add self-tests")
Link: http://lkml.kernel.org/r/20170201225629.C3070852@viggo.jf.intel.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>