Fix a missing include of <string>

gold/ChangeLog:

2019-06-07  Martin Liska  <mliska@suse.cz>

	* errors.h: Include string.

gold/ChangeLog

+2019-06-10  Martin Liska  <mliska@suse.cz>
+
+	* errors.h: Include string.
+
 2019-05-10  Joshua Oreman  <oremanj@hudson-trading.com>
 
 	PR gold/21066

gold/errors.h

@@ -24,6 +24,7 @@
 #define GOLD_ERRORS_H
 
 #include <cstdarg>
+#include <string>
 
 #include "gold-threads.h"