Mention C++11 in meta/libraries.json

There is no way to indicate required version of C++ standard,
but it can be mentioned in the description and displayed in
the boost.org/libs listings, see details at
https://lists.boost.org/Archives/boost/2019/11/247385.php

meta/libraries.json

@@ -6,7 +6,7 @@
         "Hailin Jin",
         "Christian Henning"
     ],
-    "description": "Generic Image Library",
+    "description": "(C++11) Generic Image Library",
     "category": [
         "Algorithms",
         "Containers",