[IA64] ia64 does not need umount2() syscall

ia64 doesn't have old and new versions of the umount system call.
It just has the new version.

Fixes this build warning:
<stdin>:395:2: warning: #warning syscall umount2 not implemented

Signed-off-by: Tony Luck <[e-mail]>

arch/ia64/include/asm/unistd.h

···
 #define __IGNORE_utime		/* utimes() */
 #define __IGNORE_getpgrp	/* getpgid() */
 #define __IGNORE_vfork		/* clone() */
+#define __IGNORE_umount2	/* umount() */
 
 #define __ARCH_WANT_SYS_RT_SIGACTION
 #define __ARCH_WANT_SYS_RT_SIGSUSPEND