Fix a bunch of compiler warnings.

Change-Id: Iacc3beff295d4f4f6334ef598b10c73de509b3c0
diff --git a/internal/ceres/block_jacobi_preconditioner.h b/internal/ceres/block_jacobi_preconditioner.h
index 91cfedd..51f2655 100644
--- a/internal/ceres/block_jacobi_preconditioner.h
+++ b/internal/ceres/block_jacobi_preconditioner.h
@@ -37,7 +37,7 @@
 namespace ceres {
 namespace internal {
 
-class CompressedRowBlockStructure;
+struct CompressedRowBlockStructure;
 class LinearOperator;
 class SparseMatrix;