common: Reverse deleted include

common: Reverse deleted include

Reversing 1 delete from
https://github.com/ceph/ceph/commit/09d3f546b32cdf126a1246899a0b0a3eb25282d8

Clang trips over this:
home/jenkins/workspace/ceph-master/src/common/ConfUtils.cc:94:19: error: implicit instantiation of undefined template 'std::__1::basic_ostringstream<char, std::
__1::char_traits<char>, std::__1::allocator<char> >'
    ostringstream oss;
                  ^
/usr/include/c++/v1/iosfwd:123:32: note: template is declared here
    class _LIBCPP_TEMPLATE_VIS basic_ostringstream;
                               ^

Signed-off-by: Willem Jan Withagen <wjw@digiware.nl>