Fix(Abc_ObjFanoutId): add missing inline function

[zzhou612]

In macro Abc_ObjForEachFanoutId:

#define Abc_ObjForEachFanoutId( pObj, iFanout, i )                                                 \
    for ( i = 0; (i < Abc_ObjFanoutNum(pObj)) && (((iFanout) = Abc_ObjFanoutId(pObj, i)), 1); i++ )

Function Abc_ObjFanoutId has never been declared / defined. Therefore I add the missing inline function Abc_ObjFanoutId in abc.h, following Abc_ObjFaninId.