Ignore backward compatibility macros in apibuild.py

Ignore backward compatibility macros in apibuild.py

This fixes this three warnings from the parser by allowing the parser
to ignore some macros in the same way as it can ignore functions.

Parsing ./../include/libvirt/libvirt.h
Misformatted macro comment for _virSchedParameter
 Expecting '* _virSchedParameter:' got '* virSchedParameter:'
Misformatted macro comment for _virBlkioParameter
 Expecting '* _virBlkioParameter:' got '* virBlkioParameter:'
Misformatted macro comment for _virMemoryParameter
 Expecting '* _virMemoryParameter:' got '* virMemoryParameter:'