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/network_manager_main.cpp b/network_manager_main.cpp
index 38fe673..b0d6d2e 100644
--- a/network_manager_main.cpp
+++ b/network_manager_main.cpp
@@ -11,7 +11,9 @@
 #include <fstream>
 #include <functional>
 #include <memory>
+#ifdef SYNC_MAC_FROM_INVENTORY
 #include <nlohmann/json.hpp>
+#endif
 #include <phosphor-logging/elog-errors.hpp>
 #include <phosphor-logging/log.hpp>
 #include <sdbusplus/bus.hpp>