[libc++] [test] Add missing required headers to midpoint.integer.pass.cpp

This change authored by Paolo Torres <t-pator@microsoft.com>

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@358698 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed