Adjust svc_vc_null() definition to avoid clang 15 warning

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

sys/rpc/svc_vc.c:1078:12: error: a function declaration without a prototype is deprecated in all versions of C [-Werror,-Wstrict-prototypes]
svc_vc_null()
           ^
            void

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

MFC after: 3 days