Fix junit_reporter XML output

[tim2000]

I came across this error during Jenkins JUnit integration of my benchmarks:

org.xml.sax.SAXParseException; systemId: file:/data/jenkins/jobs/CoreEngine-PT-Tests/builds/23/performance-reports/JUnit/perf_result.xml; lineNumber: 7; columnNumber: 4; The element type "property" must be terminated by the matching end-tag "".

This commit have fixed the issue. You may not accept it since I have only tested it with JUnit Jenkins parser and it may not works with other parsers.