meson: Change completions installation directory

[tesuji]

To be able to use completion with zsh, completion script must be in
the fpath variable. On Ubuntu, these paths are:

% print -ln $fpath
/usr/local/share/zsh/site-functions
/usr/share/zsh/vendor-functions
/usr/share/zsh/vendor-completions
/usr/share/zsh/functions/Calendar
/usr/share/zsh/functions/Chpwd
/usr/share/zsh/functions/<.. redacted ..>                               

On Arch Linux, these paths are:

% print -ln $fpath
/usr/local/share/zsh/site-functions
/usr/share/zsh/site-functions
/usr/share/zsh/functions/Calendar
/usr/share/zsh/functions/Chpwd
/usr/share/zsh/functions/<.. redacted ..>              

So if r2 users install radare2 with prefix=/usr/local, completion will not work
on both Ubuntu and Arch Linux. In this case, users have to edit fpath or link them to
/usr/local/share/zsh/site-functions. In case of users install radare2 with prefix=/usr,
completion will work on Arch Linux.

With this PR, users on both distro could use completion when installing with
prefix=/usr/local. However, Ubuntu users might need to manually edit
fpath themselves.