qt6.qtbase: fixup build by adding #include paths

https://hydra.nixos.org/build/256292509/nixlog/1/tail

pkgs/development/libraries/qt-6/modules/qtbase.nix

···
     "-framework GSS"
   ]);
 
-  env.NIX_CFLAGS_COMPILE = "-DNIXPKGS_QT_PLUGIN_PREFIX=\"${qtPluginPrefix}\"";
+  env.NIX_CFLAGS_COMPILE = "-DNIXPKGS_QT_PLUGIN_PREFIX=\"${qtPluginPrefix}\""
+    # These flags are in glib-2.0.pc but somehow don't get applied automatically.
+    + lib.optionalString (!stdenv.hostPlatform.isMinGW)
+      " -I${glib.dev}/include/glib-2.0 -I${glib.out}/lib/glib-2.0/include";
 
   outputs = [ "out" "dev" ];