Appveyor CI: build artifacts for x64

Fixes https://github.com/KhronosGroup/glslang/issues/1396
diff --git a/.appveyor.yml b/.appveyor.yml
index 4463fa6..55ce618 100644
--- a/.appveyor.yml
+++ b/.appveyor.yml
@@ -40,7 +40,7 @@
 
 build_script:
   - mkdir build && cd build
-  - cmake .. -DCMAKE_INSTALL_PREFIX=install
+  - cmake -G "Visual Studio 12 2013 Win64" -DCMAKE_INSTALL_PREFIX=install ..
   - cmake --build . --config %CONFIGURATION% --target install
 
 test_script: