builtin/init-db.c: eliminate -Wformat warning on Solaris

builtin/init-db.c: eliminate -Wformat warning on Solaris

On Solaris systems we'd warn about an implicit cast of mode_t when we
printed things out with the %d format. We'd get this warning under GCC
4.6.0 with Solaris headers:

    builtin/init-db.c: In function ‘separate_git_dir’:
    builtin/init-db.c:354:4: warning: format ‘%d’ expects argument of type ‘int’, but argument 2 has type ‘mode_t’ [-Wformat]

We've been doing this ever since v1.7.4.1-296-gb57fb80. Just work
around this by adding an explicit cast.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>