[CMake] install ldl.hh and spqr.hh

This will install the new headers ldl.hh and spqr.hh that were introduced by merge request !8 (merged).

dune/istl/CMakeLists.txt

@@ -16,6 +16,7 @@ install(FILES
ilusubdomainsolver.hh
io.hh
istlexception.hh
+   ldl.hh
matrix.hh
matrixindexset.hh
matrixmarket.hh
@@ -39,6 +40,7 @@ install(FILES
solver.hh
solvers.hh
solvertype.hh
+   spqr.hh
superlu.hh
supermatrix.hh
umfpack.hh