meson support: remove code warnings 1

This commit contains code changes necessary to support the increased
warning level from Meson builds. Most changes are for unused variables.

To keep the review size manageable, this commit contains only monitor
and presence changes (and top-level json_config.hpp).

Signed-off-by: Mike Capps <mikepcapps@gmail.com>
Change-Id: I7280b512c54e8d5aeba3300764a239f3dcbab14d
diff --git a/monitor/fan.cpp b/monitor/fan.cpp
index fa8b392..7bd1542 100644
--- a/monitor/fan.cpp
+++ b/monitor/fan.cpp
@@ -506,7 +506,7 @@
     }
 }
 
-void Fan::powerStateChanged(bool powerStateOn)
+void Fan::powerStateChanged([[maybe_unused]] bool powerStateOn)
 {
 #ifdef MONITOR_USE_JSON
     if (powerStateOn)