[gdb/tui] Fix fingerprint for cmd-only layout

[gdb/tui] Fix fingerprint for cmd-only layout

I added a cmd-only layout:
...
(gdb) tui new-layout cmd cmd 1
...
and set it:
...
(gdb) layout cmd
...
which gave me the expect result: only the cmd window in the screen.

However, after going back to layout src:
...
(gdb) layout src
...
I got a source window with only one line in it, and the cmd window taking most
of the screen.

I traced this back to tui_set_layout, where for both the old and the new
layout the fingerprint of the cmd window in the layout is taken.  If the
fingerprint is the same, an effort will be done to preserve the command
window size.

The fingerprint is "VC" for both the old (cmd) and new (src) layouts, which
explains the behaviour.

I think this is essentially a bug in the finger print calculation, and it
should be "C" for the cmd layout.

Fix this by not adding a V or H in the fingerprint if the list size is one.

Tested on x86_64-linux.

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