fortran: Change PMPI_Aint_{add,diff} to functions.

They were incorrectly changed to subroutines in only `pmpi`
in 258d1aa1607.

Strictly speaking, this change involves binary incompatibility.
But nobody used these subroutines and nobody will be affected because
these subroutines were useless (didn't return a calculated value).

Signed-off-by: KAWASHIMA Takahiro <t-kawashima@jp.fujitsu.com>

ompi/mpi/fortran/use-mpi-f08/mod/pmpi-f08-interfaces.F90

@@ -2484,21 +2484,23 @@ end function  PMPI_Wtime_f08
 end interface PMPI_Wtime
 
 interface PMPI_Aint_add
-subroutine PMPI_Aint_add_f08(base,diff)
+function  PMPI_Aint_add_f08(base,diff)
    use :: mpi_f08_types, only : MPI_ADDRESS_KIND
    implicit none
    INTEGER(MPI_ADDRESS_KIND) :: base
    INTEGER(MPI_ADDRESS_KIND) :: diff
-end subroutine PMPI_Aint_add_f08
+   INTEGER(MPI_ADDRESS_KIND) :: PMPI_Aint_add_f08
+end function PMPI_Aint_add_f08
 end interface PMPI_Aint_add
 
 interface PMPI_Aint_diff
-subroutine PMPI_Aint_diff_f08(addr1,addr2)
+function  PMPI_Aint_diff_f08(addr1,addr2)
    use :: mpi_f08_types, only : MPI_ADDRESS_KIND
    implicit none
    INTEGER(MPI_ADDRESS_KIND) :: addr1
    INTEGER(MPI_ADDRESS_KIND) :: addr2
-end subroutine PMPI_Aint_diff_f08
+   INTEGER(MPI_ADDRESS_KIND) :: PMPI_Aint_diff_f08
+end function PMPI_Aint_diff_f08
 end interface PMPI_Aint_diff
 
 interface  PMPI_Abort