include: apply chibi-scheme bug fix ahead of time

A missing `extern` keyword in a header file causes multiple instances
of the sexp_primitive_opcodes variable (and probably others) to be
generated at compile time.  That's bad because the instance that is
used by libchibi-scheme stays NULL, resulting in a segmentation fault.

See http://code.google.com/p/chibi-scheme/issues/detail?id=197