Add a missing #include to a header.

Add a missing #include to a header.

This was masked by lucky #include ordering in the .cpp files and
uncovered when we moved to the canonical ordering because the primary
header was included first (yay!). Unfortunately, I can't build this
locally so took a build-bot iteration to find it.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@304789 91177308-0d34-0410-b5e6-96231b3b80d8