pstore: Add hsize argument in write_buf call of pstore_ftrace_call

Incorporate the addition of hsize argument in write_buf callback
of pstore. This was forgotten in

    6bbbca73
    pstore: Pass header size in the pstore write callback

Causing a build failure when ftrace and pstore are enabled.

Signed-off-by: Aruna Balakrishnaiah <aruna@linux.vnet.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>