[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: NTony Luck <tony.luck@intel.com>

arch/ia64/include/asm/unistd.h

@@ -311,7 +311,7 @@
 #define __NR_preadv			1319
 #define __NR_pwritev			1320
 #define __NR_rt_tgsigqueueinfo		1321
-#define __NR_rt_recvmmsg		1322
+#define __NR_recvmmsg			1322
 
 #ifdef __KERNEL__