mpi: Provide the function prototype of __udiv_qrnnd.
* mpi/longlong.h [__alpha] (__udiv_qrnnd): Add the prototype. [__hppa] (__udiv_qrnnd): Likewise. [__sparc__] (__udiv_qrnnd): Likewise.
- GnuPG-bug-id: T7721
- Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>