unit_struct.h is not included in release tarballs

[nieder]

When building and testing 2.0.0 or 3.0.0b2, compiling fails like this:

gcc -DHAVE_CONFIG_H -I. -I../../src   -I/sw/include -DYY_NO_INPUT -DYY_NO_UNPUT -D_FORTIFY_SOURCE=2 -Wall -Werror -g -O2 -fstack-protector-strong -Werror -MT test_clean_lower.o -MD -MP -MF .deps/test_clean_lower.Tpo -c -o test_clean_lower.o test_clean_lower.c
test_clean_lower.template:17:10: fatal error: 'unit_struct.h' file not found
#include "unit_struct.h"
         ^~~~~~~~~~~~~~~

The tarball is missing this file, even though it's in git.