Fix opencv builds by adding a missing header,
https://crbug.com/1059030.

Github PR: https://github.com/opencv/opencv/pull/16740

diff --git a/modules/core/include/opencv2/core/cvstd.inl.hpp b/modules/core/include/opencv2/core/cvstd.inl.hpp
index 4a85bfd..37ad1e6 100644
--- a/modules/core/include/opencv2/core/cvstd.inl.hpp
+++ b/modules/core/include/opencv2/core/cvstd.inl.hpp
@@ -46,6 +46,7 @@
 
 #include <complex>
 #include <ostream>
+#include <sstream>
 
 //! @cond IGNORED