gdb: Move definitions of std::string overloads in ui_out to the header

These methods are just trivial wrappers around the versions accepting
a char pointer.  By moving them to the header the compiler can inline
them.

gdb/ChangeLog:

	* ui-out.c (ui_out::field_string): Move to ui-out.h.
	(ui_out::text): Ditto.
	* ui-out.h (class ui_out): Add definitions of
	ui_out::field_string and ui_out::text which were previously
	defined in ui-out.c.