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

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

sys/ufs/ufs/ufs_dirhash.c:1252:18: error: a function declaration without a prototype is deprecated in all versions of C [-Werror,-Wstrict-prototypes]
ufsdirhash_lowmem()
                 ^
                  void

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

MFC after: 3 days

(cherry picked from commit 08c16dd4bf6ee5649a86fa472104f89b3fbfe71a)