Fix do_bfgs_optimize.hpp to pass header tests.
Fix do_bfgs_optimize.hpp to pass header tests.

stan-dev/stan,stan-dev/math,stan-dev/math,stan-dev/math,stan-dev/math,stan-dev/math,stan-dev/math,stan-dev/stan,stan-dev/stan,stan-dev/math,stan-dev/stan,stan-dev/math,stan-dev/stan,stan-dev/math