Make tui-winsource not use breakpoint_chain

Make tui-winsource not use breakpoint_chain

That's an internal variable of breakpoint.c. Insted, use
iterate_over_breakpoints to update the breakpoint list.

gdb/ChangeLog:

2019-10-15  Christian Biesinger  <cbiesinger@google.com>

	* breakpoint.c (breakpoint_chain): Make static.
	* tui/tui-winsource.c: Call iterate_over_breakpoints instead
	of accessing breakpoint_chain.

Change-Id: Ic259b2c3a4c1f5a47f34cfd7fccbdcf274417429