[Bug-readline] _rl_search_cxt may look different to different compilation units

[Benjamin Peterson]

The layout of the _rl_search_cxt struct, defined in rlprivate.h, depends on
whether HANDLE_MULTIBYTE is defined when the struct is defined. Since
rlprivate.h doesn't include rlmbutil.h, the header responsible for defining or
not defining HANDLE_MULTIBYTE, the layout of _rl_search_cxt will depend on
whether the translation unit has included rlmbutil.h before it includes
rlprivate.h.

In particular, callback.c doesn't include rlmbutil.h but uses references
variables of type _rl_search_cxt. This could easily be problematic in the
future.

A patch to bandaid the current issue follows, but a more systematic fix like
including rlmbutil.h in rlprivate.h is probably better.