replace #include <ios> in FBString.h with lighter #include <iosfwd>

replace #include <ios> in FBString.h with lighter #include <iosfwd>

Summary:
FBString.h doesn't really use any of the facilities defined in <ios>, such as the stream manipulators. No sense dragging it into FBString.h.

This could potentially break downstream users if they are not already including the io facilities they are using. It's unlikely though because the actual stream types (std::ostream, std::istream) and stream objects (std::cout, std::cin) are defined in other headers that #include <ios>.

Reviewed By: yfeldblum, Orvid

Differential Revision: D4886133

fbshipit-source-id: 56adb93280eeeef8b09320b30fb224d4f72707bf