fix harmless inconsistency in semtimedop

this should not matter since the reality is that either all the sysv
sem syscalls are individual syscalls, or all of them are multiplexed
on the SYS_ipc syscall (depending on arch). but best to be consistent
anyway.

src/ipc/semtimedop.c

@@ -5,7 +5,7 @@
 
 int semtimedop(int id, struct sembuf *buf, size_t n, const struct timespec *ts)
 {
-#ifdef SYS_semop
+#ifdef SYS_semtimedop
 	return syscall(SYS_semtimedop, id, buf, n, ts);
 #else
 	return syscall(SYS_ipc, IPCOP_semtimedop, id, n, 0, buf, ts);