json: format output with 4-space indentation

Use `dump(4)` instead of `dump()` to generate JSON with
4-space indentation, improving readability of written files.

Change-Id: Ida39298a0110d815abc99a1fbc779f5c9378d139
Signed-off-by: George Liu <liuxiwei@ieisystem.com>
diff --git a/dump_entry.cpp b/dump_entry.cpp
index b7fa3a0..98560a2 100644
--- a/dump_entry.cpp
+++ b/dump_entry.cpp
@@ -92,7 +92,7 @@
         j["originatorType"] = originatorType();
         j["startTime"] = startTime();
 
-        os << j.dump();
+        os << j.dump(4);
     }
     catch (const std::exception& e)
     {