Windows CI: add missing cmakefile reference to build script.

appveyor.yml

@@ -21,5 +21,5 @@ build:
 build_script:
  - cd c:\projects\assimp
  - cd
- - cmake -G"Visual Studio 11" ..
+ - cmake CMakeLists.txt -G"Visual Studio 11" ..
  - msbuild /m /p:Configuration=Release /p:Platform="Win32" Assimp.sln