sadf: XML: Update XSD document so that XML header validates

sadf: XML: Update XSD document so that XML header validates

Trying to validate XML output from "sadf -Hx" fails because some
elements are expected to be found, e.g.:

$ sadf -Hx | xmllint --schema xml/sysstat.xsd -
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE sysstat PUBLIC "DTD v3.8 sysstat //EN" "http://pagesperso-orange.fr/sebastien.godard/sysstat-3.8.dtd">
<sysstat xmlns="http://pagesperso-orange.fr/sebastien.godard/sysstat" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://pagesperso-orange.fr/sebastien.godard sysstat.xsd">
	<sysdata-version>3.8</sysdata-version>
	<host nodename="linux-4.home">
		<sysname>Linux</sysname>
		<release>5.0.16-100.fc28.x86_64</release>
		<machine>x86_64</machine>
		<number-of-cpus>8</number-of-cpus>
		<file-date>2019-10-12</file-date>
		<file-utc-time>16:03:24</file-utc-time>
		<timezone>CET</timezone>
	</host>
</sysstat>
-:9: element host: Schemas validity error : Element '{http://pagesperso-orange.fr/sebastien.godard/sysstat}host': Missing child element(s). Expected is ( {http://pagesperso-orange.fr/sebastien.godard/sysstat}statistics ).
- fails to validate

Update XSD document to indicate that "statistics" and "restarts"
elements may not exist.

Signed-off-by: Sebastien GODARD <sysstat@users.noreply.github.com>