strongswan: fix up path for modprobe

strongswan uses `modprobe` to load IPSec-related kernel modules.  The
full path needs to be specified to `modprobe` for it to be able to be
found.

(cherry picked from commit 7143062172f6bad877a87c8e239f2421e0a48e2d)

pkgs/tools/networking/strongswan/default.nix

@@ -1,9 +1,10 @@
-{ stdenv, fetchurl
+{ stdenv, fetchurl, substituteAll
 , pkgconfig, autoreconfHook
 , gmp, python, iptables, ldns, unbound, openssl, pcsclite
 , openresolv
 , systemd, pam
 , curl
+, kmod
 , enableTNC            ? false, trousers, sqlite, libxml2
 , enableNetworkManager ? false, networkmanager
 , libpcap
@@ -40,6 +41,10 @@ stdenv.mkDerivation rec {
     ./ext_auth-path.patch
     ./firewall_defaults.patch
     ./updown-path.patch
+    (substituteAll {
+      src = ./modprobe-path.patch;
+      inherit kmod;
+    })
   ];
 
   postPatch = ''