handleMapPostrender returns undefined

Without this we get the following warning from the compiler:

JSC_EXPORTED_FUNCTION_UNKNOWN_RETURN_TYPE. Unable to determine return type for exported function ol.control.Control.prototype.handleMapPostrender at ../src/ol/control/control.js line 79 : 51

I'm not sure why explicitely specifying the return type is required here.
Merge pull request #708 from elemoine/user-extensions

The compilation of ol.js produces a warning