gnu: shadow: Build fix for the Hurd.

* gnu/packages/patches/shadow-hurd-pctrl.patch: New file.
* gnu/local.mk (dist_patch_DATA): Add it.
* gnu/packages/admin.scm (shadow): Use it.

gnu/local.mk

@@ -1430,6 +1430,7 @@ dist_patch_DATA =						\
   %D%/packages/patches/seahorse-gkr-use-0-on-empty-flags.patch	\
   %D%/packages/patches/seq24-rename-mutex.patch			\
   %D%/packages/patches/sharutils-CVE-2018-1000097.patch		\
+  %D%/packages/patches/shadow-hurd-pctrl.patch			\
   %D%/packages/patches/shepherd-hurd.patch			\
   %D%/packages/patches/shishi-fix-libgcrypt-detection.patch	\
   %D%/packages/patches/slim-session.patch			\

gnu/packages/admin.scm

@@ -488,6 +488,7 @@ (define-public shadow
               (uri (string-append
                     "https://github.com/shadow-maint/shadow/releases/"
                     "download/" version "/shadow-" version ".tar.xz"))
+              (patches (search-patches "shadow-hurd-pctrl.patch"))
               (sha256
                (base32
                 "0qmfq50sdhz6xilgxvinblll8j2iqfl7hwk45bq744y4plq4dbd3"))))

gnu/packages/patches/shadow-hurd-pctrl.patch

@@ -0,0 +1,16 @@
+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
+