[json] - fix jsonbuilder was not found when added to "PATH" from outside...

.... Add the same approach like done in codegenerator.mk - fixes json generation when compiling from xcode

@montellese @wsnipex for review ...

imo the right thing for the xbmc/interfaces/json-rpc/Makefile would be to use a Makefile.in and use @abs_top_srcdir@ - see xbmc/interfaces/python/Makefile but iirc someone also proposed that too.

PS: this is a build fix for all developers building from inside xcode (jenkins doesn't it uses xcodebuild which somehow doesn't seem to suffer from this issue).