vboxsf: fix building with GCC 15

vboxsf: fix building with GCC 15

Building with GCC 15 results in build error
fs/vboxsf/super.c:24:54: error: initializer-string for array of ‘unsigned char’ is too long [-Werror=unterminated-string-initialization]
   24 | static const unsigned char VBSF_MOUNT_SIGNATURE[4] = "\000\377\376\375";
      |                                                      ^~~~~~~~~~~~~~~~~~
cc1: all warnings being treated as errors

Due to GCC having enabled -Werror=unterminated-string-initialization[0]
by default. Separately initializing each array element of
VBSF_MOUNT_SIGNATURE to ensure NUL termination, thus satisfying GCC 15
and fixing the build error.

[0]: https://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html#index-Wno-unterminated-string-initialization

Signed-off-by: Brahmajit Das <brahmajit.xyz@gmail.com>
Link: https://lore.kernel.org/r/20250121162648.1408743-1-brahmajit.xyz@gmail.com
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>