[IA64] Fix cut/paste detritus from unistd.h

Build warning:
<stdin>:1523:2: warning: #warning syscall recvmmsg not implemented

Because when recvmmesg was added, the previous syscall define was
cut&pasted, and a spurious "rt_" left in the name of the define.

Signed-off-by: Tony Luck <tony.luck@intel.com>