libcamera: Declare static local variables as const where applicable

We use static local variables to indicate errors in methods that return
a const reference. The local variables can thus be const, make them so.

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

src/libcamera/log.cpp

@@ -749,7 +749,7 @@ void LogCategory::setSeverity(LogSeverity severity)
  */
 const LogCategory &LogCategory::defaultCategory()
 {
-	static LogCategory category("default");
+	static const LogCategory category("default");
 	return category;
 }