getdeps: fix xxhash build on windows

Summary:
X-link: https://github.com/facebookincubator/zstrong/pull/1015

fbthrift needs xxhash but xxhash manifest had no builder defined on windows

Add xxhash windows build support using cmake

X-link: facebook/fbthrift#624

Reviewed By: andreacampi

Differential Revision: D64592635

Pulled By: ahornby

fbshipit-source-id: 5dc558e8786fa1264ba0e89026ca750d1a285dea

Author: ahornby

build/fbcode_builder/manifests/xxhash

@@ -25,3 +25,6 @@ all
 [make.install_args]
 install
 
+[build.os=windows]
+builder = cmake
+subdir = xxHash-0.8.2/cmake_unofficial