qtwebengine: fix build failure due to -Wformat-security

ninja builds some components with -Wno-format, which is not compatible with the
default hardening setting of -Wformat-security in Nixpkgs.

pkgs/development/libraries/qt-5/modules/qtwebengine.nix

@@ -31,6 +31,10 @@ qtModule {
 
   enableParallelBuilding = true;
 
+  # ninja builds some components with -Wno-format,
+  # which cannot be set at the same time as -Wformat-security
+  hardeningDisable = [ "format" ];
+
   postPatch =
     # Patch Chromium build tools
     ''