Fix a compilation issue with LLVMs libc++

Fix a compilation issue with LLVMs libc++

Due to more restrictive include policy template<class T1, class T2> struct pair
is not already defined in std at this point. Unfortunately, a forward
declaration is invalid because it doesn't allow for instantiations:

  "error: implicit instantiation of undefined template"

Furthermore, ISO/IEC C++ 2011 Standard, section 17.6.4.2.1 prohibits such
manipulation of the namespace std.

git-svn-id: https://svn.dealii.org/branches/branch_port_to_libcxx@30189 0785d39b-7218-0410-832d-ea1e28bc413d