[lldb][Docs] Add Curses version note to build page

This explains a thing that hit me on FreeBSD because the base system
has an ncursesw at one version and I installed from pkg another
version that was simply ncurses (no wide char support).

For whatever reason, when we pass -lcurses to the linker it ends up
picking bits of both installs. This led to lldb crashing immediately
if you tried to use the `gui` command.

In a way that gave little information but I stumbled onto
https://github.com/vifm/vifm/issues/325 which is very similar.

```
ec2-user@freebsd:~/build-llvm $ ldd ./bin/lldb | grep curses
	libncursesw.so.9 => /lib/libncursesw.so.9 (0x6a515206e000)
	libncurses.so.6 => /usr/local/lib/libncurses.so.6 (0x6a5158e86000)
```

We should only see one version, and it and libpanel etc should
all have "w" or not have "w". This was not the case for my build.

What I can see from the CMake side seemed fine, it found the pkg
installed ncurses in /usr/local. Something else must decide that
-lcurses should pull in the other one.

Regardless, I don't know how to fix that but the solution for most
people is just not to add another ncurses if they already have one.
So I've added a note saying so, and how to check what your lldb
is using.

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