libcamera: log: Mark Loggable::_log() methods as const

The methods don't modify the object instance, mark them as const. This
allows using the LOG() macro from a const method of a Loggable object.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Niklas Söderlund <niklas.soderlund@ragnatech.se>
Reviewed-by: Jacopo Mondi <jacopo@jmondi.org>

src/libcamera/include/log.h

@@ -76,9 +76,10 @@ protected:
 	virtual std::string logPrefix() const = 0;
 
 	LogMessage _log(const char *file, unsigned int line,
-			LogSeverity severity);
+			LogSeverity severity) const;
 	LogMessage _log(const char *file, unsigned int line,
-			const LogCategory &category, LogSeverity severity);
+			const LogCategory &category,
+			LogSeverity severity) const;
 };
 
 LogMessage _log(const char *file, unsigned int line, LogSeverity severity);