Fixes x64 build

Author: j-ulrich

.appveyor.yml

@@ -3,7 +3,7 @@ version: "{build}"
 environment:
     CTEST_OUTPUT_ON_FAILURE: '1'
     matrix:
-        - CMAKE_GENERATOR: Visual Studio 15 2017
+        - CMAKE_GENERATOR: Visual Studio 15 2017 Win64
           QT_DIR: C:\Qt\5.11\msvc2017_64
           APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2017
           VCVARS_COMMANDLINE: '"C:\Program Files (x86)\Microsoft Visual Studio\2017\Community\VC\Auxiliary\Build\vcvars64.bat"'