Add missing #include <string> to copy.h which was causing compilation errors with MSVC.
Add missing #include <string> to copy.h which was causing compilation errors with MSVC.

--HG--
extra : convert_revision : svn%3A83215879-3e5a-4751-8c9d-778f44bb06a5/trunk%40134

jaredhoberock/thrust,marksantos/thrust,xiongzhanblake/thrust,thrust/thrust,sdalton1/thrust,zeryx/thrust,Ricardo666666/thrust,sdalton1/thrust,marksantos/thrust,jaredhoberock/thrust,GrimDerp/thrust,thvasilo/thrust,zhenglaizhang/thrust,mohamed-ali/thrust,arnabgho/thrust,thrust/thrust,jaredhoberock/thrust,dachziegel/thrust,GrimDerp/thrust,Ricardo666666/thrust,Ricardo666666/thrust,xiongzhanblake/thrust_src,thrust/thrust,andrewcorrigan/thrust-multi-permutation-iterator,mohamed-ali/thrust,marksantos/thrust,mohamed-ali/thrust,xiongzhanblake/thrust,arnabgho/thrust,egaburov/thrust,zhenglaizhang/thrust,jaredhoberock/thrust,egaburov/thrust,raygit/thrust,zhenglaizhang/thrust,arnabgho/thrust,sdalton1/thrust,thvasilo/thrust,thrust/thrust,andrewcorrigan/thrust-multi-permutation-iterator,GrimDerp/thrust,xiongzhanblake/thrust_src,andrewcorrigan/thrust-multi-permutation-iterator,sarvex/thrust,zeryx/thrust,sarvex/thrust,dachziegel/thrust,thvasilo/thrust,dachziegel/thrust,xiongzhanblake/thrust,xiongzhanblake/thrust_src,raygit/thrust,raygit/thrust,zeryx/thrust,egaburov/thrust,jaredhoberock/thrust,sarvex/thrust,thrust/thrust