Adjust local_rpcb() definition to avoid clang 15 warning

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

sys/rpc/rpcb_clnt.c:439:11: error: a function declaration without a prototype is deprecated in all versions of C [-Werror,-Wstrict-prototypes]
local_rpcb()
          ^
           void

This is because local_rpcb() 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 e87d90a9dce22003d79ae895bac5ce2d0ca029fe)