Use struct buffer in gdb_readline_no_editing_callback

Use struct buffer in gdb_readline_no_editing_callback

gdb/ChangeLog:
2016-03-09  Pedro Alves  <palves@redhat.com>

	* event-top.c: Include buffer.h.
	(gdb_readline_no_editing_callback): Use struct buffer instead
	of xrealloc.