LibreWriter doesn’t provide (so far) a smart way to set a “styled” space between the number of a footnote and the text (of that footnote).
Therefore you should add a (or more than) space in each footnote. But it would be a bad way to solve the problem.
A workaround can be to set the footnote characters (in styles) with a) a border (with an invisible color, such a s white over a white background) and b) a right padding. You can see in the following image:
