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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<div class='path'>path: <a href='/guix/log/?id=350e61bcf01c7327d9c68eb9222a855cd8174f09'>root</a>/<a href='/guix/log/tests?id=350e61bcf01c7327d9c68eb9222a855cd8174f09'>tests</a>/<a href='/guix/log/tests/civodul.key?id=350e61bcf01c7327d9c68eb9222a855cd8174f09'>civodul.key</a></div><div class='content'><table class='list nowrap'><tr class='nohover'><th class='left'>Age</th><th class='left'>Commit message (<a href='/guix/log/tests/civodul.key?id=350e61bcf01c7327d9c68eb9222a855cd8174f09&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2020-05-04 09:56:12 +0200'>2020-05-04</span></td><td><a href='/guix/commit/tests/civodul.key?id=43408e304ffb1149f35cb539b40d673d567c9116'>Add (guix openpgp).</a><span class='msg-avail'>...<span class='msg-tooltip'>* guix/openpgp.scm, tests/openpgp.scm, tests/civodul.key,
tests/dsa.key, tests/ed25519.key, tests/rsa.key,
tests/ed25519.sec: New files.
* Makefile.am (MODULES): Add guix/openpgp.scm.
(SCM_TESTS): Add tests/openpgp.scm.
(EXTRA_DIST): Add tests/*.key and tests/ed25519.sec.
</span></span></td><td>Ludovic Courtès</td></tr>