Fixing SVG icon for sETH and sUSD token

[KagamiChan]

This PR fixes token icon for sETH and sUSD.

An SVG XML should contain the ns information according to SVG standard here: https://www.w3.org/TR/SVG11/struct.html#NewDocumentOverview, which is saying:

In all cases, for compliance with the Namespaces in XML Recommendation [XML-NS], an SVG namespace declaration must be provided so that all SVG elements are identified as belonging to the SVG namespace.

Without this, the svg could not be recognized by some parsers and renderers, which might be problematic

Also I suggest we add some CI checks to make sure SVG icons are standard compliant