Fixed some unused parameters warnings

They cause problems when using -Werror in your compilation.
base-linux: Don't busywait if lx_pollpid returns 0

This happened when I killed one of the genode clients which was
tracked via an expception_handler. In this case the wait4 syscall
returned 0 and the for(;;) was looped eternally. This caused an
100% CPU utilization for the core binary.