This should be included as `<lilv/lilv.h>`, same as is done in af_lv2.c.
Forcing the extra lilv-0 breaks platforms where the include dir is
`/usr/include/lilv/lilv.h` rather than
`/usr/include/lilv-0/lilv/lilv.h`.

The new include path works for both, because the `pkg-config --cflags`
includes `-I/usr/include/lilv-0`.