Add final specifier to internal classes.

This should help the compiler devirtualize a bunch of function
calls.

https://github.com/ceres-solver/ceres-solver/issues/747

Change-Id: I9913e68d85e0e9c9f955a249cc710a657875c869

internal/ceres/implicit_schur_complement.h

@@ -89,7 +89,7 @@ class BlockSparseMatrix;
 // RightMultiply (and the LeftMultiply) methods are not thread safe as
 // they depend on mutable arrays used for the temporaries needed to
 // compute the product y += Sx;
-class CERES_NO_EXPORT ImplicitSchurComplement : public LinearOperator {
+class CERES_NO_EXPORT ImplicitSchurComplement final : public LinearOperator {
  public:
   // num_eliminate_blocks is the number of E blocks in the matrix
   // A.