Compilation succeeds when forward declarations lack 'obliv' qualifier.

[jackdoerner]

If one writes an obliv function, and then writes a forward declaration of that function that lacks the obliv qualifier in a header that is #included, it seems that obliv-c happily compiles without error. However, if code from another file attempts to call said function, having also included the header, a segfault will result: the caller will not pass an enable bit, whereas the callee will expect to receive one. Suggested remedy: detect non-matching forward declaration, and issue an error.