waitid: linux ABI takes 5 args, not 4
waitid: linux ABI takes 5 args, not 4

The POSIX waitid() takes 4 args, but the Linux one takes 5 args, so make
sure we stuff the 5th arg with a NULL.  Otherwise garbage gets randomly
passed up and considering this is a pointer, that's baaaad.

Signed-off-by: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@gentoo.org>

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