Don't overwrite $prefix if already bound

I have the following in .tmux.conf:

```conf
unbind C-b
set -g prefix C-Space

# quickly switch between panes
bind C-Space select-pane -t :.+

set -g @tpm_plugins "              \
  tmux-plugins/tpm                 \
  tmux-plugins/tmux-sensible       \
"
run-shell ~/.tmux/plugins/tpm/tpm
```

When tmux-sensible gets loaded, it overwrites my existing C-Space binding.