fcitx-qt5: fix build

[ericsagnes]

Things done

• Tested using sandboxing (nix-build --option build-use-chroot true or nix.useChroot on NixOS)
• Built on platform(s)
  • NixOS
  • OS X
  • Linux
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Fits CONTRIBUTING.md.

---

Without this change, the install phase fails with the following message:

CMake Error at platforminputcontext/cmake_install.cmake:50 (file):
  file INSTALL cannot copy file
  "/tmp/nix-build-fcitx-qt5-1.0.5.drv-0/fcitx-qt5-1.0.5/build/platforminputcontext/libfcitxplatforminputcontextplugin.so"
  to
  "/nix/store/5ixiz6661qki5a3crlv4q74hc6l8za7g-qtbase-5.5.1/lib/qt5/plugins/platforminputcontexts/libfcitxplatforminputcontextplugin.so".
Call Stack (most recent call first):
  cmake_install.cmake:39 (include)

@ttuegel I am not yet familiar with multiple outputs, is this the right way to address this problem?

[ttuegel]

Looks good to me!