[mpfa][props] add missing include
If a problem was created where only mpfa was used, i.e. only the mpfa properties were included, there was an error from the property system which stem from this missing include. We never ran into this problem because we always have several discretization methods being tested with one problem file.
To reproduce this error choose any test and comment out the includes of tpfa or box. Then try to compile the mpfa test which should give an error. With this commit the error should disappear.