Add a missing include

Change-Id: I3060e80a69374f7a2db31121715d0d892a54d311

internal/ceres/parallel_invoke.h

diff --git a/internal/ceres/parallel_invoke.h b/internal/ceres/parallel_invoke.h
index f076631..73c3aea 100644
--- a/internal/ceres/parallel_invoke.h
+++ b/internal/ceres/parallel_invoke.h
@@ -40,6 +40,7 @@
 #include <type_traits>
 
 #include "absl/log/check.h"
+#include "ceres/context_impl.h"
 
 namespace ceres::internal {