Only use sd_journal_* if we really are outputing to journald

We check that the device and inode numbers are actually the same as the
JOURNAL_STREAM value, instead of just checking that the value exists.

This fixes the logger unit tests

src/logger/logger.hpp

@@ -9,6 +9,7 @@
  */
 
 #include <memory>
+#include <string>
 #include <iostream>
 #include <fstream>
 #include <sstream>