Enable Yocto control of the USING_ENTITY_MANAGER_DECORATORS

The USING_ENTITY_MANAGER_DECORATORS compile switch is hard coded to
enabled in a header file. The changes here allow the feature to be
controlled in recipes via PACKAGECONFIG entries in bbappend files.

Tested:
Applied https://gerrit.openbmc.org/c/openbmc/openbmc/+/63434
Modified internal bbappend file to PACKAGECONFIG:remove
entity-manager-decorators
Confirmed the code guarded by the ifdef no longer compiled
Removed change to bbappend file
Confirmed the code guarded by the ifdef was restored.

Change-Id: I4f3072451f893fcb5edaa478d0546253e7005b9f
Signed-off-by: Johnathan Mantey <johnathanx.mantey@intel.com>
diff --git a/meson.build b/meson.build
index 2b82689..9ea6e1c 100644
--- a/meson.build
+++ b/meson.build
@@ -75,6 +75,7 @@
   'update-functional-on-fail'  : '-DUPDATE_FUNCTIONAL_ON_FAIL',
   'dynamic-sensors'            : '-DFEATURE_DYNAMIC_SENSORS',
   'dynamic-sensors-write'      : '-DFEATURE_DYNAMIC_SENSORS_WRITE',
+  'entity-manager-decorators'  : '-DUSING_ENTITY_MANAGER_DECORATORS',
   'hybrid-sensors'             : '-DFEATURE_HYBRID_SENSORS',
   'sensors-cache'              : '-DFEATURE_SENSORS_CACHE',
   'sel-logger-clears-sel'      : '-DFEATURE_SEL_LOGGER_CLEARS_SEL',