Change this filename back (i.e., revert r31375).

It's a singular filename because there's only 1 interface in the
file.  Also, r31375 missed updating the name in a few places, and
broke the build for compilers that supported the mpi_f08 interface.

This commit was SVN r31389.

The following SVN revision numbers were found above:
  r31375 --> open-mpi/ompi@fe1935de14

ompi/mpi/fortran/use-mpi-f08/Makefile.am

@@ -709,7 +709,7 @@ libmpi_usempif08_la_SOURCES = \
         mpi-f-interfaces-bind.h \
         pmpi-f-interfaces-bind.h \
         attr-fn-f08-callback-interfaces.h \
-        conversion-fn-null-f08-interfaces.h \
+        conversion-fn-null-f08-interface.h \
         mpi-f08.F90 \
         constants.h \
         constants.c