AutoDeleter: add Get method for FileDescriptorCloser

This allows to use FileDescriptorCloser as unique pointer for file descriptor.

Change-Id: I4c768fafba6ed35658b2fdb075b9b547f53bc8da
Reviewed-on: https://review.haiku-os.org/c/haiku/+/2495
Reviewed-by: Jérôme Duval <jerome.duval@gmail.com>

headers/private/shared/AutoDeleter.h

@@ -254,6 +254,11 @@ struct FileDescriptorCloser {
 		SetTo(-1);
 	}
 
+	inline int Get()
+	{
+		return fDescriptor;
+	}
+
 	inline int Detach()
 	{
 		int descriptor = fDescriptor;