frame.h: Forward-declare struct ui_out

frame.h: Forward-declare struct ui_out

Fixes this failure when building in C mode.  I think it's relevant for master
as well, since it's a good practice to include (or forward-declare) what you
use.

In file included from ../../binutils-gdb/gdb/gdbarch.h:38:0,
                 from ../../binutils-gdb/gdb/defs.h:653,
                 from ../../binutils-gdb/gdb/dictionary.c:23:
../../binutils-gdb/gdb/frame.h:710:48: warning: ‘struct ui_out’ declared inside parameter list will not be visible outside of this definition or declaration
 extern void print_stack_frame_to_uiout (struct ui_out *uiout,

gdb/ChangeLog:

	* frame.h: Forward-declare struct ui_out.