[Serialization] Add missing include
[Serialization] Add missing include

forward decl is not sufficient for destroying a unique_ptr<MemoryBuffer>.

git-svn-id: ffe668792ed300d6c2daa1f6eba2e0aa28d7ec6c@355816 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/clang,apple/swift-clang,apple/swift-clang,llvm-mirror/clang,apple/swift-clang,llvm-mirror/clang,llvm-mirror/clang,llvm-mirror/clang,llvm-mirror/clang,llvm-mirror/clang,llvm-mirror/clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,llvm-mirror/clang,llvm-mirror/clang,apple/swift-clang