Add an include guard to manifold_test_utils.h

Including the header twice redefines its matchers. Guard the header.

Change-Id: I7064d2caf438a787372e45a446913773edccfa55

include/ceres/manifold_test_utils.h

diff --git a/include/ceres/manifold_test_utils.h b/include/ceres/manifold_test_utils.h
index 43fe3e1..765b27a 100644
--- a/include/ceres/manifold_test_utils.h
+++ b/include/ceres/manifold_test_utils.h
@@ -28,6 +28,9 @@
 //
 // Author: sameeragarwal@google.com (Sameer Agarwal)
 
+#ifndef CERES_PUBLIC_MANIFOLD_TEST_UTILS_H_
+#define CERES_PUBLIC_MANIFOLD_TEST_UTILS_H_
+
 #include <algorithm>
 #include <cmath>
 #include <limits>
@@ -384,3 +387,5 @@
 }
 
 }  // namespace ceres
+
+#endif  // CERES_PUBLIC_MANIFOLD_TEST_UTILS_H_