Fixed omitted includes.

The includes of the respective C++11 standard headers were
missing. This issue becomes visible only when including one of those
headers before including a standard C++ header. As the namespace std::
simply does not exist without including a standard header, these two
headers produced errors on clang.

From: TILL TOENSHOFF <toenshoff@me.com>
Review: https://reviews.apache.org/r/16912