logging: fix compilation error on older C++ compilers

logging: fix compilation error on older C++ compilers

Summary:
Add an explicit `std::move()` around the return value of
`logConfigToDynamic()`.  This explicit move is required pre-C++14,
but is not required after DR 1579:
http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#1579

Newer versions of g++ and clang no longer require the move.  However gcc
versions earlier than 5.0 do require it.

Reviewed By: yfeldblum

Differential Revision: D6466447

fbshipit-source-id: 1b15934490d4966e9b3e5612e0e2ecbb43c979ca