chiaki: fix build on darwin

Darwin requires qtmacextra in the buildInputs, otherwise compilation
fails with:

  CMake Error at /nix/store/05nmn6v1hrk56zqs61s2awvykha51fx9-qtbase-5.12.7-dev/lib/cmake/Qt5/Qt5Config.cmake:28 (find_package):
    By not providing "FindQt5MacExtras.cmake" in CMAKE_MODULE_PATH this project
    has asked CMake to find a package configuration file provided by
    "Qt5MacExtras", but CMake did not find one.

    Could not find a package configuration file provided by "Qt5MacExtras" with
    any of the following names:

      Qt5MacExtrasConfig.cmake
      qt5macextras-config.cmake

    Add the installation prefix of "Qt5MacExtras" to CMAKE_PREFIX_PATH or set
    "Qt5MacExtras_DIR" to a directory containing one of the above files.  If
    "Qt5MacExtras" provides a separate development package or SDK, be sure it
    has been installed.
  Call Stack (most recent call first):
    gui/CMakeLists.txt:7 (find_package)

  -- Configuring incomplete, errors occurred!
  See also "/tmp/nix-build-chiaki-2.0.1.drv-0/chiaki/build/CMakeFiles/CMakeOutput.log".

pkgs/games/chiaki/default.nix

@@ -1,5 +1,5 @@
 { lib, mkDerivation, fetchgit
-, cmake, ffmpeg, libevdev, libopus, udev, qtbase, qtmultimedia
+, cmake, ffmpeg, libevdev, libopus, udev, qtbase, qtmacextras, qtmultimedia
 , qtsvg , pkg-config, protobuf , python3Packages, SDL2, stdenv }:
 
 with stdenv.lib;
@@ -18,7 +18,8 @@ mkDerivation rec {
     cmake pkg-config protobuf python3Packages.python python3Packages.protobuf
   ];
   buildInputs = [ ffmpeg libopus qtbase qtmultimedia qtsvg protobuf SDL2 ]
-    ++ optionals stdenv.hostPlatform.isLinux [ libevdev udev];
+    ++ optionals stdenv.hostPlatform.isLinux [ libevdev udev]
+    ++ optionals (stdenv.isDarwin) [ qtmacextras ];
 
   doCheck = true;
   installCheckPhase = "$out/bin/chiaki --help";