https://www.iconfinder.com/icons/1269936/linkedin_social_social_media_icon#size=16

_includes/footer.html

...
<ul class="social-media-list">
  {% if site.linkedin_username %}
  <li>
    {% include icon-linkedin.html username=site.linkedin_username %}
  </li>
  {% endif %}
...

_includes/icon-linkedin.html

<a href="https://linkedin.com/in/{{ include.username }}"><span class="icon icon--linkedin">{% include icon-linkedin.svg %}</span><span class="username">{{ include.username }}</span></a>

_includes/icon-linkedin.svg (https://www.iconfinder.com/icons/1269936/linkedin_social_social_media_icon#size=16)

<?xml version="1.0" ?><!DOCTYPE svg  PUBLIC '-//W3C//DTD SVG 1.1//EN'  'http://www.w3.org/Graphics/SVG/1.1/DTD/svg11.dtd'><svg id="Capa_1" style="enable-background:new 0 0 430.1 430.1;" version="1.1" viewBox="0 0 430.1 430.1" xml:space="preserve" xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink"><g><g><path d="M430.1,261.5v159h-92.2V272.2c0-37.3-13.3-62.7-46.7-62.7c-25.5,0-40.6,17.1-47.3,33.7     c-2.4,5.9-3.1,14.2-3.1,22.5v154.9h-92.2c0,0,1.2-251.3,0-277.3h92.2v39.3c-0.2,0.3-0.4,0.6-0.6,0.9h0.6v-0.9     c12.3-18.9,34.1-45.8,83.1-45.8C384.6,136.7,430.1,176.4,430.1,261.5z M52.2,9.6C20.6,9.6,0,30.3,0,57.5c0,26.6,20,47.9,51,47.9     h0.6c32.2,0,52.2-21.3,52.2-47.9C103.1,30.3,83.7,9.6,52.2,9.6z M5.5,420.6h92.2V143.2H5.5V420.6z" id="LinkedIn"/></g></g></svg>