Fix a bunch of compiler warnings.

Change-Id: Iacc3beff295d4f4f6334ef598b10c73de509b3c0
diff --git a/internal/ceres/visibility_based_preconditioner.h b/internal/ceres/visibility_based_preconditioner.h
index 888c65e..3246fb8 100644
--- a/internal/ceres/visibility_based_preconditioner.h
+++ b/internal/ceres/visibility_based_preconditioner.h
@@ -69,7 +69,7 @@
 
 class BlockRandomAccessSparseMatrix;
 class BlockSparseMatrixBase;
-class CompressedRowBlockStructure;
+struct CompressedRowBlockStructure;
 class SchurEliminatorBase;
 
 // This class implements three preconditioners for Structure from