network_manager: JSON is only used for SYNC_MAC

We don't include the header when this feature isn't enabled

Change-Id: I99f0ad5d40378bf381c878bce4539e6790f76335
Signed-off-by: William A. Kennington III <wak@google.com>
diff --git a/util.cpp b/util.cpp
index 34df649..5c5e427 100644
--- a/util.cpp
+++ b/util.cpp
@@ -15,7 +15,9 @@
 #include <filesystem>
 #include <fstream>
 #include <list>
+#ifdef SYNC_MAC_FROM_INVENTORY
 #include <nlohmann/json.hpp>
+#endif
 #include <phosphor-logging/elog-errors.hpp>
 #include <phosphor-logging/log.hpp>
 #include <stdexcept>