Fix compilation with GSL.
Fix compilation with GSL.

As of 2d311f0f48 the conversion constructor from Tensor to SymmetricTensor is
explicit.

danshapero/dealii,sairajat/dealii,sairajat/dealii,sairajat/dealii,naliboff/dealii,sairajat/dealii,sairajat/dealii,naliboff/dealii,naliboff/dealii,naliboff/dealii,danshapero/dealii,danshapero/dealii,danshapero/dealii,sairajat/dealii,naliboff/dealii,danshapero/dealii,danshapero/dealii,danshapero/dealii,naliboff/dealii,sairajat/dealii,naliboff/dealii