rB55c281415b67 removed 'BLENDER_RENDER' as a COMPAT_ENGINE but the
cycles addon checks for this in its get_panels() function.
Adding this back for now.
(Not sure if it makes sense to do a cleanup and replace 'BLENDER_RENDER'
with 'CYCLES' throughout ui scripts? -- cycles addon as well as other
addons would probably require some adjustments...)