Address the cpplint findings of json_view's comparisons

compare.hpp includes <string> (build/include_what_you_use).

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

include/nlohmann/detail/view/compare.hpp

@@ -10,6 +10,7 @@
 
 #include <algorithm> // sort, stable_sort
 #include <cstddef> // size_t
+#include <string> // string
 #include <utility> // move, pair
 #include <vector> // vector