Add a missing <cstdint> to block_structure.h

Change-Id: Ic192eb61b87726e5e939a28b00842da4e0f2b30a

internal/ceres/block_structure.h

diff --git a/internal/ceres/block_structure.h b/internal/ceres/block_structure.h
index 5761a1f..b5218c0 100644
--- a/internal/ceres/block_structure.h
+++ b/internal/ceres/block_structure.h
@@ -38,6 +38,7 @@
 #ifndef CERES_INTERNAL_BLOCK_STRUCTURE_H_
 #define CERES_INTERNAL_BLOCK_STRUCTURE_H_
 
+#include <cstdint>
 #include <vector>
 #include "ceres/internal/port.h"