entity_name: cleanup headers

Drop unused headers, add missing explicit header.

Change-Id: I35d94ba205cef677e59620c24e27ec0bffba7743
Signed-off-by: Patrick Venture <venture@google.com>
diff --git a/entity_name.cpp b/entity_name.cpp
index 30b8c28..c070c84 100644
--- a/entity_name.cpp
+++ b/entity_name.cpp
@@ -22,14 +22,11 @@
 #include <cstring>
 #include <filesystem>
 #include <fstream>
+#include <map>
 #include <nlohmann/json.hpp>
 #include <phosphor-logging/elog-errors.hpp>
 #include <phosphor-logging/log.hpp>
-#include <regex>
-#include <sstream>
 #include <string>
-#include <system_error>
-#include <unordered_map>
 #include <vector>
 #include <xyz/openbmc_project/Common/error.hpp>