Shared/API/c/WKBase.h must be processed before being installed.

There's an important #if clause that only gets set during WebKit build,
but makes WebKitNix library users including WKBase.h to fail.
WKBase.h must be preprocessed to guarantee the code inside the #if
clause will be used.

Also, WKAPICastNix.h should not be installed.

Reviewed by: Rafael Brandão