os: sanity check in Reporter::Xml_generator

With this check in place, one can safely construct an 'Xml_generator'
even if the report is disabled. This relieves the user of the reporter
from the need to distinguish enabled from disabled reports.

repos/os/include/os/reporter.h

@@ -146,7 +146,8 @@ class Genode::Reporter : Noncopyable
 				                      reporter._xml_name.string(),
 				                      func)
 			{
-				reporter._conn->report.submit(used());
+				if (reporter.enabled())
+					reporter._conn->report.submit(used());
 			}
 		};
 };