Fix missing include <utility> for std::forward/std::move.

This will fix [Test output: BP x86_64 C++11 - type_erasure - test_is_subconcept / clang-linux-3.5~c14_libc++ ](http://www.boost.org/development/tests/master/developer/output/BP%20x86_64%20C++11-boost-bin-v2-libs-type_erasure-test-test_is_subconcept-test-clang-linux-3-5~c14_libc++-debug-debug-symbols-off.html).