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 <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Reviewed-by: Hans de Goede <[e-mail]>
Signed-off-by: Christian Brauner <[e-mail]>

fs/vboxsf/super.c

···
 
 #define VBOXSF_SUPER_MAGIC 0x786f4256 /* 'VBox' little endian */
 
-static const unsigned char VBSF_MOUNT_SIGNATURE[4] = "\000\377\376\375";
+static const unsigned char VBSF_MOUNT_SIGNATURE[4] = { '\000', '\377', '\376',
+						       '\375' };
 
 static int follow_symlinks;
 module_param(follow_symlinks, int, 0444);