xtensa: fix kconfig unmet dependency warning for HAVE_FUTEX_CMPXCHG

XTENSA should only select HAVE_FUTEX_CMPXCHG when FUTEX is
set/enabled. This prevents a kconfig warning.

WARNING: unmet direct dependencies detected for HAVE_FUTEX_CMPXCHG
  Depends on [n]: FUTEX [=n]
  Selected by [y]:
  - XTENSA [=y] && !MMU [=n]

Fixes: d951ba21 ("xtensa: nommu: select HAVE_FUTEX_CMPXCHG")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Max Filippov <jcmvbkbc@gmail.com>
Cc: Chris Zankel <chris@zankel.net>
Cc: linux-xtensa@linux-xtensa.org
Message-Id: <20210526070337.28130-1-rdunlap@infradead.org>
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>