event_pipe, test: explicitly ignore write() return value

Some compilers are very picky, but we really aren't interested in the
return value.

src/event_pipe.c

@@ -159,5 +159,6 @@ void event_pipe_emit_fast(enum pipe_event event)
 	assert((unsigned)event < PIPE_EVENT_MAX);
 
 	pipe_events[event] = true;
-	(void)write(event_pipe[1], "", 1);
+
+	G_GNUC_UNUSED ssize_t nbytes = write(event_pipe[1], "", 1);
 }