[gdb/tui] Fix Wmaybe-uninitialized warning in tui-winsource.c

[gdb/tui] Fix Wmaybe-uninitialized warning in tui-winsource.c

When compiling with CFLAGS/CXXFLAGS="-O0 -g -Wall" and using g++ 11.0.0, we
run into:
...
src/gdb/tui/tui-winsource.c: In function \
  'void tui_update_all_breakpoint_info(breakpoint*)':
src/gdb/tui/tui-winsource.c:427:58: warning: '<unknown>' may be used \
  uninitialized [-Wmaybe-uninitialized]
  427 |   for (tui_source_window_base *win : tui_source_windows ())
      |                                                          ^
In file included from src/gdb/tui/tui-winsource.c:38:
src/gdb/tui/tui-winsource.h:236:30: note: by argument 1 of type \
  'const tui_source_windows*' to 'tui_source_window_iterator \
  tui_source_windows::begin() const' declared here
  236 |   tui_source_window_iterator begin () const
      |                              ^~~~~
src/gdb/tui/tui-winsource.c:427:58: note: '<anonymous>' declared here
  427 |   for (tui_source_window_base *win : tui_source_windows ())
      |                                                          ^
...

The warning doesn't make sense for an empty struct, PR gcc/96295 has been
filed about that.

For now, work around the warning by defining a default constructor.

Build on x86_64-linux.

gdb/ChangeLog:

2020-07-23  Tom de Vries  <tdevries@suse.de>

	PR tui/26282
	* tui/tui-winsource.h (struct tui_source_windows::tui_source_windows):
	New default constructor.