Update WindowsBuild.md

Enable SourceKit builds by default even on Windows, this has been ported to Windows though the test suite is not yet ready.

Author: compnerd

docs/WindowsBuild.md

@@ -160,6 +160,7 @@ cmake -G Ninja^
  -DCMAKE_CXX_FLAGS:STRING="-Wno-c++98-compat -Wno-c++98-compat-pedantic"^
  -DCMAKE_EXE_LINKER_FLAGS:STRING="/INCREMENTAL:NO"^
  -DCMAKE_SHARED_LINKER_FLAGS:STRING="/INCREMENTAL:NO"^
+ -DSWIFT_BUILD_SOURCEKIT=ON^
  -DSWIFT_INCLUDE_DOCS=OFF^
  -DSWIFT_PATH_TO_CMARK_SOURCE="S:\cmark"^
  -DSWIFT_PATH_TO_CMARK_BUILD="S:\b\cmark"^