Context should be exported

Change-Id: I451a7f471a6b93d1d32dc003adaa3c5c315fef9f

include/ceres/context.h

diff --git a/include/ceres/context.h b/include/ceres/context.h
index 87fba0f..6c6e8f4 100644
--- a/include/ceres/context.h
+++ b/include/ceres/context.h
@@ -41,7 +41,7 @@
 // Problems, either serially or in parallel. When using it with multiple
 // Problems at the same time, they may end up contending for resources
 // (e.g. threads) managed by the Context.
-class CERES_NO_EXPORT Context {
+class CERES_EXPORT Context {
  public:
   Context();
   Context(const Context&) = delete;