Small compile fix to context_impl

* Add missing header include for <string>, affects CUDA builds.
* Fix typo

Change-Id: I82ca6eb180b85a7b85966c36d6ec59ce78726a96

internal/ceres/context_impl.h

diff --git a/internal/ceres/context_impl.h b/internal/ceres/context_impl.h
index 7c49ba6..ed6bedc 100644
--- a/internal/ceres/context_impl.h
+++ b/internal/ceres/context_impl.h
@@ -34,7 +34,9 @@
 // This include must come before any #ifndef check on Ceres compile options.
 // clang-format off
 #include "ceres/internal/config.h"
-// clanf-format on
+// clang-format on
+
+#include <string>
 
 #include "ceres/context.h"
 #include "ceres/internal/disable_warnings.h"