FixedArray internal struct was not respecting Eigen alignment.

Added EIGEN_MAKE_ALIGNED_OPERATOR_NEW to the struct so that
the Jet members are aligned. This fixes an eigen assert in
autodiff_test reported by multiple Ceres users.

Thanks Koichi Akabe & Stephan Kassemeyer

Change-Id: Id3574e926deffa57d205dddaa9d08389b5dc33a8

include/ceres/internal/fixed_array.h

diff --git a/include/ceres/internal/fixed_array.h b/include/ceres/internal/fixed_array.h
index 02d3b71..aa1722c 100644
--- a/include/ceres/internal/fixed_array.h
+++ b/include/ceres/internal/fixed_array.h
@@ -34,6 +34,7 @@
 
 #include <cstddef>
 #include <glog/logging.h>
+#include "Eigen/Core"
 #include "ceres/internal/macros.h"
 #include "ceres/internal/manual_constructor.h"
 
@@ -132,6 +133,7 @@
   // of this code will be broken.
   struct InnerContainer {
     T element;
+    EIGEN_MAKE_ALIGNED_OPERATOR_NEW
   };
 
   // How many elements should we store inline?