Fix an instantiation.

The existing instantiation works, but is not in line with the actual
declaration of the function. This may or may not be the problem with
the failing tests here http://cdash.kyomu.43-1.org/viewTest.php?onlyfailed&buildid=10664
that result from a missing instantiation.
Merge pull request #49 from bangerth/fix-instantiation

Fix an instantiation.