iio-buffer-utils: Really fix sorting of channels

As per the documentation of g_ptr_array_sort
"Note that the comparison function for g_ptr_array_sort() doesn't take the
pointers from the array as arguments, it takes pointers to the pointers in
the array"
So the arguments to the function
	compare_channel_index (gconstpointer a, gconstpointer b)
needs to be dereferenced to get pointer to iio_channel_info.

The error was included in the fixup for commit 984803b by
the maintainer.

Closes #99

src/iio-buffer-utils.c

@@ -196,8 +196,8 @@ channel_info_free (iio_channel_info *ci)
 static int
 compare_channel_index (gconstpointer a, gconstpointer b)
 {
-	const iio_channel_info *info_1 = a;
-	const iio_channel_info *info_2 = b;
+	const iio_channel_info *info_1 = *(iio_channel_info **) a;
+	const iio_channel_info *info_2 = *(iio_channel_info **) b;
 
 	return (int) (info_1->index - info_2->index);
 }