chiaki: fix build on darwin
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".

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