Added the KWLet header to the appropriate copy build phases.

I compiled the Kiwi-iOS target to include the framework in my project and `KWLet.h` was missing from the headers directory.

I based my assumptions on what build phases to include it in based on `KWBlock.h`.

KWLet.h is included in Copy Headers and Copy Files build phases of the iOS targets, and the Copy Headers of the OSX target.