JSON-RPC: automatically create JSON schema API description

Right now when editing the JSON schema API description of our JSON-RPC API we need to adjust one of the *.json files in xbmc/interfaces/json-rpc and we need to make the same changes in xbmc/interfaces/json-rpc/ServiceDescription.h which is read by the JSON-RPC implementation. But editing that file by hand is very tedious (which is why we have the separate *.json files in the first place). @topfs2 and I have written a tool to do that conversion and it's located at https://github.com/Montellese/jsd_builder. It takes the *.json files, a license.txt and a version.txt file to build the ServiceDescription.h. But right now this is a manual task.

This adds the necessary bits to the win32 build system to automatically create ServiceDescription.h during the build (similar to how it's done for the python code generation).
Would be nice if someone with the necessary linux bootstrap/depends/Makefile foo could add the necessary bits to also automate this in the linux buildsystem. IMO it can be done the same way as for the python code generation (i.e. during bootstrapping IIRC).

Obviously this is Gotham + 1.