AppVeyor: Add -j2 to cmake build command.

This should theoretically speed up Windows compilation.

extra/appveyor/appveyor.yml

@@ -38,7 +38,7 @@ before_build:
 - cmake -G "%GENERATOR%" -H. -Bbuild
 
 build_script:
-- cmake --build build --target PACKAGE --config Release
+- cmake --build build --target PACKAGE --config Release -j2
 
 after_build: