Remove qualifier from function return type in tst-svc_register.c.

Building the testsuite with -Wextra produces a warning in
sunrpc/tst-svc_register.c for a useless qualifier on a function return
type.  This patch removes that qualifier.

Tested for x86_64.

	* sunrpc/tst-svc_register.c (rpcbind_address): Remove qualifier
	from function return type.