fwbuilder: disable blanket -Werror

`-Werror` flag usually causes build failures due to minor changes in
compiler versions. They might be useful for developers themselves but
are rarely useful for distributions.

last time `fwbuilder` failed the build against newer `libxml2`:

    libfwbuilder/src/fwbuilder/XMLTools.cpp: In static member function 'static void libfwbuilder::XMLTools::initXMLTools()':
    libfwbuilder/src/fwbuilder/XMLTools.cpp:222:18: error: 'int xmlInitMemory()' is deprecated [-Werror=deprecated-declarations]
      222 |     xmlInitMemory();
          |     ~~~~~~~~~~~~~^~

pkgs/tools/security/fwbuilder/default.nix

@@ -20,6 +20,13 @@ stdenv.mkDerivation rec {
     hash = "sha256-j5HjGcIqq93Ca9OBqEgSotoSXyw+q6Fqxa3hKk1ctwQ=";
   };
 
+  postPatch = ''
+    # Avoid blanket -Werror as it triggers on any minor compiler
+    # warnings like deprecated functions or invalid indentat8ion.
+    # Leave fixing these problems to upstream.
+    substituteInPlace CMakeLists.txt --replace ';-Werror;' ';'
+  '';
+
   nativeBuildInputs = [
     cmake
     ninja
@@ -32,11 +39,6 @@ stdenv.mkDerivation rec {
     qtwayland
   ];
 
-  NIX_CFLAGS_COMPILE = [
-    "-Wno-error=misleading-indentation"
-    "-Wno-error=deprecated-declarations"
-  ];
-
   meta = with lib; {
     description = "GUI Firewall Management Application";
     longDescription = ''