build: add EXTRA_API_HEADERS variable

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

src/CMakeLists.txt

@@ -220,6 +220,7 @@ set(api_headers
     ${PROJECT_SOURCE_DIR}/src/lib/core/clock.h
     ${PROJECT_SOURCE_DIR}/src/box/decimal.h
     ${PROJECT_SOURCE_DIR}/src/lua/decimal.h
+    ${EXTRA_API_HEADERS}
 )
 rebuild_module_api(${api_headers})