Since this font driver is by default enabled in `modules.cfg` and its
availability controlled by `FT_CONFIG_OPTION_HVF`, we need to provide some
stubs so that HVF fonts can be correctly rejected if there is no HVF library
available.
Fixes issue #1409.
* src/hvf/hvfdrv.c (hvf_driver_class): Provide `hvf_face_init` and
`hvf_slot_load_glyph` unconditionally.
* src/hvf/hvfload.c, src/hvf/hvfload.h (hvf_slot_load_glyph)
[!FT_CONFIG_OPTION_HVF]: Define stub.
* src/hvf/hvfobjs.c, src/hvf/hvfobjs.h (hvf_face_init)
[!FT_CONFIG_OPTION_HVF]: Define stub.
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