fix implicit "any" type in index.d.ts
Created by: slhck
These functions need an explicit response type to prevent warnings like the following:
index.d.ts (350,9): 'registerLicenseRequestFilter', which lacks return-type annotation, implicitly has an 'any' return type. index.d.ts (351,9): 'registerLicenseResponseFilter', which lacks return-type annotation, implicitly has an 'any' return type. index.d.ts (352,9): 'unregisterLicenseRequestFilter', which lacks return-type annotation, implicitly has an 'any' return type. index.d.ts (353,9): 'unregisterLicenseResponseFilter', which lacks return-type annotation, implicitly has an 'any' return type