libcamera: base: log: Pass dynamic prefix through

Use move construction to essentially pass through the string
returned by `Loggable::logPrefix()` to avoid an unnecessary copy.

Signed-off-by: Barnabás Pőcze <pobrn@protonmail.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Jacopo Mondi <jacopo.mondi@ideasonboard.com>

include/libcamera/base/log.h

@@ -64,7 +64,7 @@ class LogMessage
 public:
 	LogMessage(const char *fileName, unsigned int line,
 		   const LogCategory &category, LogSeverity severity,
-		   const std::string &prefix = std::string());
+		   std::string prefix = {});
 	~LogMessage();
 
 	std::ostream &stream() { return msgStream_; }