eiskaltdcpp: mark linux only

The darwin build fails and other platforms will probably run into
similar issues.

    Determining if the Q_WS_QWS exist failed with the following output:
    Change Dir: /tmp/nix-build-eiskaltdcpp-2.2.10.drv-0/source/build/CMakeFiles/CMakeTmp

    Run Build Command:"/nix/store/qfag24z6xsr6jkyi8gb4cv62rp945rbk-gnumake-4.2.1/bin/make" "cmTC_14e67/fast"
    /nix/store/qfag24z6xsr6jkyi8gb4cv62rp945rbk-gnumake-4.2.1/bin/make -f CMakeFiles/cmTC_14e67.dir/build.make CMakeFiles/cmTC_14e67.dir/build
    make[1]: Entering directory '/private/tmp/nix-build-eiskaltdcpp-2.2.10.drv-0/source/build/CMakeFiles/CMakeTmp'
    Building CXX object CMakeFiles/cmTC_14e67.dir/CheckSymbolExists.cxx.o
    /nix/store/2gdwhdzhy4iwkp7fh8v6gy6nxj1zi9pv-clang-wrapper-5.0.2/bin/clang++   -I/nix/store/8dzqilmdr0p3qmmrxh51xk7wli6grm0i-qt-4.8.7/include -F/nix/store/8dzqilmdr0p3qmmrxh51xk7wli6grm0i-qt-4.8.7/lib  -std=c++0x -pipe -Wformat -Werror=format-security  -arch x86_64   -o CMakeFiles/cmTC_14e67.dir/CheckSymbolExists.cxx.o -c /tmp/nix-build-eiskaltdcpp-2.2.10.drv-0/source/build/CMakeFiles/CMakeTmp/CheckSymbolExists.cxx
    /tmp/nix-build-eiskaltdcpp-2.2.10.drv-0/source/build/CMakeFiles/CMakeTmp/CheckSymbolExists.cxx:8:19: error: use of undeclared identifier 'Q_WS_QWS'
      return ((int*)(&Q_WS_QWS))[argc];
                  ^

/cc ZHF #45961