Fix cast in TUI_DISASM_WIN

I noticed that the TUI_DISASM_WIN macro cast the disassembly window to
a base type, rather than its correct type.  This patch fixes this
oversight.

2020-02-22  Tom Tromey  <tom@tromey.com>

	* tui/tui-data.h (TUI_DISASM_WIN): Cast to tui_disasm_window.

Change-Id: Ied3dbac9ef3dc48ceb9e0850fe4ada3c316dd769