Fix compiler warning.

It works with signed and unsigned  integers.

include/nlohmann/detail/output/serializer.hpp

@@ -442,7 +442,7 @@ class serializer
             return;
         }
 
-        const bool is_negative = (x <= 0) and (x != 0);  // see issue #755
+        const bool is_negative = !(x>=0);  // see issue #755
         std::size_t i = 0;
 
         while (x != 0)