zdtm/static/fdt_shared: Fix buffer size

The TEST_STRING "Hello World!" has 13 bytes size. Therefore the 10 bytes
used for the buffer size are not sufficient.

Fix this and use sizeof() to be on the safe side.

Signed-off-by: Michael Holzheu <holzheu@linux.vnet.ibm.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>