libcamera: signal: Replace object.h inclusion with forward declatation

The signal.h header doesn't need to include object.h. Replace it with a
forward declaration, and instead include object.h in source files that
require it. It can speed up compilation a little bit, but more
importantly avoids unintended dependencies from the Signal class to the
Object class to be added later as the compiler will catch them.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Milan Zamazal <mzamazal@redhat.com>

test/message.cpp

@@ -11,6 +11,7 @@
 #include <thread>
 
 #include <libcamera/base/message.h>
+#include <libcamera/base/object.h>
 #include <libcamera/base/thread.h>
 
 #include "test.h"