Restore some windows-tdep.c code

Restore some windows-tdep.c code

When I removed init_w32_command_list, I weirdly neglected to see if it
was called anywhere else.  This patch restores the function, which is
called from windows-nat.c.  Sorry about the breakage.

Is it possible to have a windows-native gdb that isn't also using
windows-tdep?

Anyway, I'm checking this in.

gdb/ChangeLog
2020-04-18  Tom Tromey  <tom@tromey.com>

	* windows-tdep.c (init_w32_command_list)
	(w32_prefix_command_valid): Restore.
	(_initialize_windows_tdep): Call init_w32_command_list.