etsf-io: added missing mpi variant (#23083)

The variant was mentioned in a depends_on directive,
but never declared

var/spack/repos/builtin/packages/etsf-io/package.py

@@ -2,8 +2,6 @@
 # Spack Project Developers. See the top-level COPYRIGHT file for details.
 #
 # SPDX-License-Identifier: (Apache-2.0 OR MIT)
-
-
 from spack import *
 
 
@@ -22,6 +20,8 @@ class EtsfIo(Package):
 
     version('1.0.4', sha256='3140c2cde17f578a0e6b63acb27a5f6e9352257a1371a17b9c15c3d0ef078fa4')
 
+    variant('mpi', default=True, description='Add MPI support')
+
     depends_on("netcdf-fortran")
     depends_on("hdf5+mpi~cxx", when='+mpi')  # required for NetCDF-4 support