sys-apps/iproute2: backport setns check fix

Could have led to a build w/o setns support.

Signed-off-by: Sam James <sam@gentoo.org>

sys-apps/iproute2/files/iproute2-5.19.0-setns-check.patch

@@ -0,0 +1,22 @@
+https://github.com/shemminger/iproute2/commit/d5fe96ab70928508f072a47449e9b641e46de323
+
+From d5fe96ab70928508f072a47449e9b641e46de323 Mon Sep 17 00:00:00 2001
+From: Khem Raj <raj.khem@gmail.com>
+Date: Wed, 10 Aug 2022 22:34:40 -0700
+Subject: [PATCH] configure: Define _GNU_SOURCE when checking for setns
+
+glibc defines this function only as gnu extention
+
+Signed-off-by: Khem Raj <raj.khem@gmail.com>
+Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
+--- a/configure
++++ b/configure
+@@ -191,6 +191,7 @@ check_ipt_lib_dir()
+ check_setns()
+ {
+     cat >$TMPDIR/setnstest.c <<EOF
++#define _GNU_SOURCE
+ #include <sched.h>
+ int main(int argc, char **argv)
+ {
+

sys-apps/iproute2/iproute2-5.19.0-r2.ebuild

@@ -52,6 +52,7 @@ PATCHES=(
 	#"${FILESDIR}"/${PN}-5.1.0-portability.patch
 	"${FILESDIR}"/${PN}-5.7.0-mix-signal.h-include.patch
 	"${FILESDIR}"/${P}-musl.patch
+	"${FILESDIR}"/${P}-setns-check.patch
 	"${FILESDIR}"/${PN}-default-color-auto.patch
 )