libcamera: signal: Fix compilation on gcc

gcc complains that the recently template specialisation of the
SlotBase::match() method is invalid as it is in a non-namespace scope.
Luckily this can easily be fixed by removing the specialisation for the
fully specialised implementation. As an additional safety measure, using
std::enable_if<!std::is_same<Object, T>> on the template implementation
ensures that it will never be called for an Object instance.

Fixes: 56c2e65300 ("libcamera: signal: Fix Object handling in multiple inheritance cases")
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Niklas Söderlund <niklas.soderlund@ragnatech.se>

include/libcamera/signal.h

@@ -27,9 +27,8 @@ public:
 		: obj_(obj), object_(object) {}
 	virtual ~SlotBase() {}
 
-	template<typename T>
+	template<typename T, typename std::enable_if<!std::is_same<Object, T>::value>::type * = nullptr>
 	bool match(T *obj) { return obj == obj_; }
-	template<>
 	bool match(Object *object) { return object == object_; }
 
 	void disconnect(SignalBase *signal);