custom.py: Add a function to convert .gir to .gir.h
custom.py: Add a function to convert .gir to .gir.h

This is used by openwebrtc-girepository and openwebrtc

EricssonResearch/cerbero,multipath-rtp/cerbero,ramaxlo/cerbero,sdroege/cerbero,ramaxlo/cerbero,ford-prefect/cerbero,AlertMe/cerbero,AlertMe/cerbero,EricssonResearch/cerbero,shoreflyer/cerbero,superdump/cerbero,shoreflyer/cerbero,shoreflyer/cerbero,ford-prefect/cerbero,AlertMe/cerbero,superdump/cerbero,ramaxlo/cerbero,ikonst/cerbero,superdump/cerbero,sdroege/cerbero,multipath-rtp/cerbero,EricssonResearch/cerbero,multipath-rtp/cerbero,EricssonResearch/cerbero,sdroege/cerbero,sdroege/cerbero,shoreflyer/cerbero,multipath-rtp/cerbero,AlertMe/cerbero,multipath-rtp/cerbero,ramaxlo/cerbero,ford-prefect/cerbero,ikonst/cerbero,ikonst/cerbero,EricssonResearch/cerbero,superdump/cerbero,AlertMe/cerbero,sdroege/cerbero,ikonst/cerbero,ikonst/cerbero,shoreflyer/cerbero,ford-prefect/cerbero,ramaxlo/cerbero