medusa: pull upstream fix for fno-common toolchains

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

    $ nix build -L --impure --expr 'with import ./. {}; medusa.overrideAttrs (oa: {   NIX_CFLAGS_COMPILE = (["-fno-common"] ++ [oa.NIX_CFLAGS_COMPILE or ""]); })'
    ...
    ld: ../medusa-trace.o:/build/source/src/modsrc/../medusa.h:69:
      multiple definition of `iErrorLevel'; cvs.o:/build/source/src/modsrc/../medusa.h:69: first defined here

pkgs/tools/security/medusa/default.nix

@@ -1,4 +1,4 @@
-{ lib, stdenv, fetchFromGitHub, pkg-config, freerdp, openssl, libssh2 }:
+{ lib, stdenv, fetchFromGitHub, fetchpatch, pkg-config, freerdp, openssl, libssh2 }:
 
 stdenv.mkDerivation rec {
   pname = "medusa-unstable";
@@ -11,6 +11,16 @@ stdenv.mkDerivation rec {
     sha256 = "0njlz4fqa0165wdmd5y8lfnafayf3c4la0r8pf3hixkdwsss1509";
   };
 
+  patches = [
+    # Pull upstream fix for -fno-common tollchains like gcc-10:
+    #  https://github.com/jmk-foofus/medusa/pull/36
+    (fetchpatch {
+      name = "fno-common.patch";
+      url = "https://github.com/jmk-foofus/medusa/commit/a667656ad085b3eb95309932666c250d97a92767.patch";
+      sha256 = "01marqqhjd3qwar3ymp50y1h2im5ilgpaxk7wrc2kcxgmzvbdfxc";
+    })
+  ];
+
   outputs = [ "out" "man" ];
 
   configureFlags = [ "--enable-module-ssh=yes" ];