We need to filter out platform-specific sources from the files that we
use with the introspection scanner.

In this specific case I could have moved the gtkclipboard-quartz.c and
the gtkdnd-quartz.c files out of the $(gtk_base_c_sources) variable, but
doing a filter-out on the variable itself is more resilient in case we
eventually add files and we forget about the result.