sig.c: Ignore return code from write(2) to pipe.

We get warnings nowadays about this.  These are annoying.  The only
thing that can go wrong is that the pipe is already full, so the write
will block.  But if that happens, our job is already done: the select(2)
loop will certainly wake up soon because the pipe is full of stuff to be
read.  So ignoring the error here is harmless.

Insert a stupid bodge to shut the compiler up.