[krbdev.mit.edu #7953] krb5.h included after gssapi.h fails

[Greg Hudson via RT]

I cannot reproduce this with 1.12.1 (I had tried some version of master 
previously) and that test program.  Tom also cannot reproduce it.  So, I 
don't know what's going on.

It would be a problem if some public header file defined KRB5_CALLCONV 
without KRB5_CALLCONV_C, but as far as I can tell that never happens.