Avoid including sys/prctl.h on the Hurd.

Upstream status: Not submitted.

--- shadow-4.8.1/libmisc/idmapping.c.orig	2020-03-07 16:32:05.000000000 -0500
+++ shadow-4.8.1/libmisc/idmapping.c	2020-03-07 16:32:27.000000000 -0500
@@ -36,8 +36,8 @@
 #include <stdio.h>
 #include "prototypes.h"
 #include "idmapping.h"
-#include <sys/prctl.h>
 #if HAVE_SYS_CAPABILITY_H
+#include <sys/prctl.h>
 #include <sys/capability.h>
 #endif