[index] CMake: add missing reference to clangSerialization library.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@293463 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Index/CMakeLists.txt b/lib/Index/CMakeLists.txt
index 8f51ccb..c9fbfaf 100644
--- a/lib/Index/CMakeLists.txt
+++ b/lib/Index/CMakeLists.txt
@@ -24,5 +24,6 @@
   clangFormat
   clangFrontend
   clangRewrite
+  clangSerialization
   clangToolingCore
   )