IA64: Add cmpxchg.h to exported userspace headers

commit 98e4cff73a18af27f0d40d0b0d37f105dfc1994a upstream.

Fixes klibc build on ia64 after 85f8f7759e418c814ee2ceacf73eddb9bed39492.

Cc: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: maximilian attems <max@stro.at>
Signed-off-by: Tony Luck <tony.luck@intel.com>
Cc: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>