qt59.qtwebkit: fix build on Darwin

Why is qtmultimedia only needed on Darwin? Why does it only fix 5.9, not
5.11? These things I do not know. What I do know is that, for some
reason, this makes qt59.qtwebkit build on Darwin.

I think the reason it hasn't also fixed 5.11 might be something to do
with the version of qtmultimedia, but I don't know enough about Qt or
cmake to figure it out. The error when trying to build qt511.qtwebkit
(with or without these changes) is:

    CMake Error at Source/cmake/OptionsQt.cmake:739 (find_package):
      Could not find a package configuration file provided by "Qt5Multimedia"
      (requested version 5.2.0) with any of the following names:

        Qt5MultimediaConfig.cmake
        qt5multimedia-config.cmake

      Add the installation prefix of "Qt5Multimedia" to CMAKE_PREFIX_PATH or set
      "Qt5Multimedia_DIR" to a directory containing one of the above files.  If
      "Qt5Multimedia" provides a separate development package or SDK, be sure it
      has been installed.
    Call Stack (most recent call first):
      Source/cmake/WebKitCommon.cmake:50 (include)
      CMakeLists.txt:137 (include)

    -- Configuring incomplete, errors occurred!
    See also "/tmp/nix-build-qtwebkit-5.212-alpha-01-26-2018.drv-0/source/build/CMakeFiles/CMakeOutput.log".
    See also "/tmp/nix-build-qtwebkit-5.212-alpha-01-26-2018.drv-0/source/build/CMakeFiles/CMakeError.log".