ci: make module.h generation command more specific

`make api` generates `src/module.h`. `make module_api` is the target for
compiling a library for testing of the module API:
`test/app-tap/module_api.so`. It depends on the `api` target.

Both works for generating `module.h`, but `make api` don't do extra
unneeded actions.

NO_DOC=Not a user visible change.
NO_TEST=It is CI workflow. It would be too strong to test each part of
        the testing/deployment infrastructure.
NO_CHANGELOG=Not a user visible change.

.github/workflows/publish-module-api-doc.yaml

@@ -48,7 +48,7 @@ jobs:
       - name: Build module API documentation using Doxygen
         run: |
             cmake .
-            make module_api
+            make api
             sed -e "s%@PROJECT_.\+_DIR@/%%" Doxyfile.API.in > Doxyfile.API
             doxygen Doxyfile.API