openfoam: modified flex to be a build + link dependency (#15917)

fixes #15863

Since openfoam-org needs FlexInclude.h "flex" needs to
be also a link dependency.

var/spack/repos/builtin/packages/openfoam-org/package.py

@@ -84,7 +84,7 @@ class OpenfoamOrg(Package):
 
     depends_on('mpi')
     depends_on('zlib')
-    depends_on('flex',  type='build')
+    depends_on('flex')
     depends_on('cmake', type='build')
 
     # Require scotch with ptscotch - corresponds to standard OpenFOAM setup