The module API header (module.h) is generated automatically by
extracting everything between '\cond public' and '\endcond public'
from headers listed in the api_headers CMake variable. We're
planning to extend the module API in Tarantool EE. To achieve that,
this commit includes all files listed in the new CMake variable
EXTRA_API_HEADERS into api_headers.

Needed for https://github.com/tarantool/tarantool-ee/issues/265

NO_DOC=build
NO_TEST=build
NO_CHANGELOG=build