Add the missing <array> header to fixed_array.h

Change-Id: I1f48bd6eab0c6b189d01c68d4940a586a968581f

include/ceres/internal/fixed_array.h

diff --git a/include/ceres/internal/fixed_array.h b/include/ceres/internal/fixed_array.h
index 6e86e9d..69af2ea 100644
--- a/include/ceres/internal/fixed_array.h
+++ b/include/ceres/internal/fixed_array.h
@@ -31,6 +31,7 @@
 #define CERES_PUBLIC_INTERNAL_FIXED_ARRAY_H_
 
 #include <algorithm>
+#include <array>
 #include <cstddef>
 #include <memory>
 #include <tuple>