[clang-fuzzer] Add explicit dependency on clangSerialization for clangHandleCXX after rC348907

This library was breaking my -DBUILD_SHARED_LIBS=1 build. rC348915 seemed to miss this case.

As this seems an "obvious" fix, I am committing without pre-commit review as
per the LLVM developer policy.

llvm-svn: 348929