zsh completion: make $tmp local

The tmp variable is conflicting with the $tmp variable in
${^fpath}/_main_complete(N) and which is used to complete all the
functions, and causing an error: command not found: for whatever was in
$tmp (which in this case is the last value in $words[@])

making it local fixes this.

Signed-off-by: Daniel Wallace <danielwallace@gtmanfred.com>