Adjust authnone_create() definition to avoid clang 15 warning

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

sys/rpc/auth_none.c:106:16: error: a function declaration without a prototype is deprecated in all versions of C [-Werror,-Wstrict-prototypes]
authnone_create()
               ^
                void

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

MFC after: 3 days