ia64: Fixup asm/cmpxchg.h

The empty asm/cmpxchg.h file that was provided as a temporary build fix
for the asm/system.h disintgration build problem should really include
<asm/intrinsics.h> to make definitions of xchg() and cmpxchg()
available.

Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>