Work around gcc warning about function pointers conversions.

I got finally tired of seeing

    Source/DOH/fio.c: In function 'DohEncoding':
    Source/DOH/fio.c:51: warning: ISO C forbids conversion of function pointer to object pointer type
    Source/DOH/fio.c: In function 'encode':
    Source/DOH/fio.c:75: warning: ISO C forbids conversion of object pointer to function pointer type
    Source/DOH/base.c: In function 'DohCall':
    Source/DOH/base.c:952: warning: ISO C forbids conversion of object pointer to function pointer type

every time I build SWIG, so I've made a patch to work around them. The code should be exactly equivalent to the old version but doesn't result in these gcc warnings (didn't test with clang though).