bt2: EnumerationFieldType: rename append_mapping() -> add_mapping()

We're not _appending_ anything in bt2.EnumerationFieldType here, as
opposed to bt2.StructureFieldType (which contains ordered fields), so
the method should be named add_mapping(), like a Python set has the
add() method, whereas a Python list has the append() method.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>