package/openmpi: fix fortran build

Fix the following build failure with fortran raised since bump to
version 4.0.0 in commit 366e7f1e and
https://github.com/open-mpi/ompi/commit/99730f798b7aef4c87d282aa24b98916fea45bb6:

checking size of Fortran type(test_mpi_handle)... (cached) 4
checking alignment of Fortran type(test_mpi_handle)... configure: error: Can not determine alignment of type(test_mpi_handle) when cross-compiling

Fixes:
 - http://autobuild.buildroot.org/results/86ffde2f67ffc0bfaeebe72fe742a5c241bc580b



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>