stone-phaser: fix `gcc-13` build failure

Without the change build fails on `master` as
https://hydra.nixos.org/build/247541110:

In file included from ./ui/Cairo++.h:3,
from ui/components/KnobSkin.hpp:2,
from ui/components/KnobSkin.cpp:1:
./ui/Color.h:4:5: error: 'uint8_t' does not name a type
4 | uint8_t r, g, b, a;
| ^~~~~~~

pkgs/applications/audio/stone-phaser/default.nix

···
   postPatch = ''
     patch -d dpf -p 1 -i "$src/resources/patch/DPF-bypass.patch"
     patchShebangs ./dpf/utils/generate-ttl.sh
+
+    # Fix gcc-13 build failure due to missing includes
+    sed -e '1i #include <cstdint>' -i plugins/stone-phaser/ui/Color.h
   '';
 
   installFlags = [ "PREFIX=$(out)" ];