Fix py-tui.c build problem

Fix py-tui.c build problem

py-tui.c can fail to build if the ncurses development headers are not
installed, but if Python was built against ncurses.  In this case, the
Python headers will define HAVE_NCURSES_H, confusing gdb_curses.h.

This patch fixes the problem by moving this include inside
"#ifdef TUI".

gdb/ChangeLog
2020-03-31  Joel Jones  <joelkevinjones@gmail.com>

	PR tui/25597:
	* python/py-tui.c: Include gdb_curses.h inside of #ifdef TUI.