/usr/include/varnish/vmod_abi.h is in libvarnishapi-dev 4.0.2-1+deb8u1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
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#define VMOD_ABI_Version "Varnish 4.0.2 bfe7cd1"