Include the correct header for pdfork()'s prototype.

Reviewed by:	kib, markj
Obtained from:	CheriBSD
Sponsored by:	The University of Cambridge, Google Inc.
Differential Revision:	https://reviews.freebsd.org/D33988

lib/libc/sys/pdfork.c

@@ -34,7 +34,7 @@
 __FBSDID("$FreeBSD$");
 
 #include <sys/types.h>
-#include <unistd.h>
+#include <sys/procdesc.h>
 #include "libc_private.h"
 
 #pragma weak pdfork