fixing broken lxml installer for windows build server

appveyor.yml

@@ -2,7 +2,7 @@ version: '{build}'
 os: Visual Studio 2015 RC
 clone_folder: c:\projects\open62541
 before_build:
-- python -m pip install lxml
+- set STATICBUILD=true && python -m pip install lxml
 build_script:
 - cd c:\projects\open62541
 - md build