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>

include/libcamera/base/signal.h

@@ -13,10 +13,11 @@
 #include <vector>
 
 #include <libcamera/base/bound_method.h>
-#include <libcamera/base/object.h>
 
 namespace libcamera {
 
+class Object;
+
 class SignalBase
 {
 public: