treewide: Remove top-level const from return types

Top-level `const` qualifiers are not useful, so avoid them. This is done
either by simply removing the top-level `const`, or making the function
return a reference to const where that is appropriate.

Signed-off-by: Barnabás Pőcze <barnabas.pocze@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Stefan Klug <stefan.klug@ideasonboard.com>

include/libcamera/base/log.h

@@ -75,7 +75,7 @@ public:
 	const LogCategory &category() const { return category_; }
 	const std::string &fileInfo() const { return fileInfo_; }
 	const std::string &prefix() const { return prefix_; }
-	const std::string msg() const { return msgStream_.str(); }
+	std::string msg() const { return msgStream_.str(); }
 
 private:
 	LIBCAMERA_DISABLE_COPY_AND_MOVE(LogMessage)