Fix "make install" for fall-out from opm-material

Commit 38bf787 removed the local copy of (an early version of) the
opm-material module from opm-porsol, but failed to synchronise the
corresponding list of installed files.  This commit corrects that
oversight.

Insufficient testing of original change: Bård Skaflestad
Issue pointed out by: Arne Morten Kvarving
Merge pull request #61 from bska/fix-install

Fix "make install" for fall-out from opm-material