Adjust function definition in vfs_bio.c to avoid clang 15 warnings

With clang 15, the following -Werror warning is produced:

sys/kern/vfs_bio.c:3430:11: error: a function declaration without a prototype is deprecated in all versions of C [-Werror,-Wstrict-prototypes]
buf_daemon()
          ^
           void

This is because buf_daemon() is declared with a (void) argument list,
but defined with an empty argument list. Make the definition match the
declaration.

MFC after: 3 days