fwknop: pull patch pending upstream inclusion for -fno-common support

Without the change build on -fno-common tollchains fails as:

    ld: fwknop-log_msg.o:/build/source/client/log_msg.h:41: multiple definition of
      `log_level_t'; fwknop-fwknop.o:/build/source/client/log_msg.h:41: first defined here

pkgs/tools/security/fwknop/default.nix

@@ -1,4 +1,4 @@
-{ lib, stdenv, fetchFromGitHub, autoreconfHook
+{ lib, stdenv, fetchFromGitHub, fetchpatch, autoreconfHook
 , libpcap, texinfo
 , iptables
 , gnupgSupport ? true, gnupg, gpgme # Increases dependencies!
@@ -17,6 +17,16 @@ stdenv.mkDerivation rec {
     sha256 = "05kvqhmxj9p2y835w75f3jvhr38bb96cd58mvfd7xil9dhmhn9ra";
   };
 
+  patches = [
+    # Pull patch pending upstream inclusion for -fno-common tollchains:
+    #   https://github.com/mrash/fwknop/pull/319
+    (fetchpatch {
+      name = "fno-common.patch";
+      url = "https://github.com/mrash/fwknop/commit/a8214fd58bc46d23b64b3a55db023c7f5a5ea6af.patch";
+      sha256 = "0cp1350q66n455hpd3rdydb9anx66bcirza5gyyyy5232zgg58bi";
+    })
+  ];
+
   nativeBuildInputs = [ autoreconfHook ];
   buildInputs = [ libpcap texinfo ]
     ++ lib.optionals gnupgSupport [ gnupg gpgme.dev ]