hw/ppc/ppc405_boards: Initialize g_autofree pointer

hw/ppc/ppc405_boards: Initialize g_autofree pointer

Resolves the only compiler warning when building a full QEMU under Arch Linux:

  Compiling C object libqemu-ppc-softmmu.fa.p/hw_ppc_ppc405_boards.c.o
  In file included from /usr/include/glib-2.0/glib.h:114,
                   from qemu/include/glib-compat.h:32,
                   from qemu/include/qemu/osdep.h:132,
                   from ../src/hw/ppc/ppc405_boards.c:25:
  ../src/hw/ppc/ppc405_boards.c: In function ‘ref405ep_init’:
  /usr/include/glib-2.0/glib/glib-autocleanups.h:28:3: warning: ‘filename’ may be used uninitialized in this function [-Wmaybe-uninitialized]
     28 |   g_free (*pp);
        |   ^~~~~~~~~~~~
  ../src/hw/ppc/ppc405_boards.c:265:26: note: ‘filename’ was declared here
    265 |         g_autofree char *filename;
        |                          ^~~~~~~~

Signed-off-by: Bernhard Beschow <shentey@gmail.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-Id: <20220405123534.3395-1-shentey@gmail.com>
Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>