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
 
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<tr><td><span title='2019-10-23 16:40:17 +0200'>2019-10-23</span></td><td><a href='/guix/commit/tests/cve.scm?id=74afaa37d5dec1a9d1b83951529ba69d8947fb07'>cve: Rewrite to read the JSON feed instead of the XML feed.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2019-10-20 23:26:50 +0200'>2019-10-20</span></td><td><a href='/guix/commit/tests/cve.scm?id=79c03e55e26d88c8d327daced02367abe7f17f91'>tests: Avoid now-deprecated 'make-struct'.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>