Fix a bunch of compiler warnings.

Change-Id: Iacc3beff295d4f4f6334ef598b10c73de509b3c0
diff --git a/internal/ceres/compressed_row_sparse_matrix.h b/internal/ceres/compressed_row_sparse_matrix.h
index 6a9d828..10f96c0 100644
--- a/internal/ceres/compressed_row_sparse_matrix.h
+++ b/internal/ceres/compressed_row_sparse_matrix.h
@@ -42,7 +42,7 @@
 
 namespace ceres {
 
-class CRSMatrix;
+struct CRSMatrix;
 
 namespace internal {