[gdb/tui] Handle shared border in fixed-sized layout

In tui_layout_split::apply I noticed that for variable-size layouts we take
share_box into account by decreasing used_size:
...
          used_size += info[i].size;
          if (info[i].share_box)
	    --used_size;
...
but not for fixed-size layouts:
...
      if (info[i].min_size == info[i].max_size)
	available_size -= info[i].min_size;
...

Fix this by increasing available_size for fixed-size layouts with shared box.

Tested on x86_64-linux.

Approved-By: Tom Tromey <tom@tromey.com>