[libclang] Add dependency on clangSerialization to unbreak -DBUILD_SHARED_LIBS=1 build after rC349237

Frontend headers have undefined reference on the symbol `clang::PCHContainerOperations::PCHContainerOperations()` through some shared_ptr usage. Any dependents will get the undefined reference which can only be resolved by explicit dependency on clangSerialization (due to -z defs).

llvm-svn: 349259