BMessage: add AddFlat taking a const BFlattenable

It seems the const was forgotten in the BeOS version. But there is no
reason not to have it.

headers/os/app/Message.h

@@ -163,6 +163,8 @@ public:
 									const BMessage* message);
 			status_t			AddFlat(const char* name, BFlattenable* object,
 									int32 count = 1);
+			status_t			AddFlat(const char* name,
+									const BFlattenable* object, int32 count = 1);
 			status_t			AddData(const char* name, type_code type,
 									const void* data, ssize_t numBytes,
 									bool isFixedSize = true, int32 count = 1);

src/kits/app/Message.cpp

@@ -2730,6 +2730,13 @@ BMessage::AddMessage(const char* name, const BMessage* message)
 
 status_t
 BMessage::AddFlat(const char* name, BFlattenable* object, int32 count)
+{
+	return AddFlat(name, (const BFlattenable*)object, count);
+}
+
+
+status_t
+BMessage::AddFlat(const char* name, const BFlattenable* object, int32 count)
 {
 	if (object == NULL)
 		return B_BAD_VALUE;