Rename annotate_ifdef_directives to end with .py.

This allows the python doctest module to process it correctly
when invoked as:
   python -m doctest -v annotate_ifdef_directives.py

Makefile.am

@@ -477,7 +477,7 @@ version:
 
 .PHONY: autostyle-ifdefs
 autostyle-ifdefs:
-	$(PYTHON) scripts/maint/annotate_ifdef_directives $(OWNED_TOR_C_FILES)
+	$(PYTHON) scripts/maint/annotate_ifdef_directives.py $(OWNED_TOR_C_FILES)
 
 .PHONY: autostyle-ifdefs
 autostyle-operators: