Use:
<xsl:output
method="xml" version="1.0" indent="yes"
omit-xml-declaration="no" encoding="utf8"
doctype-public="-//W3C//DTD SVG 20010904//EN"
doctype-system="...DTD/svg10.dtd"
media-type="image/svg+xml"/>
to get:
<?xml version="1.0" encoding="utf8"?>
<!DOCTYPE svg
PUBLIC "-//W3C//DTD SVG 20010904//EN"
SYSTEM "...DTD/svg10.dtd">