Make ASTStructDeclaration implements NamedNode

The methods was implemented, but the interface was not declared.
Romain Lespinasse joins as a contributor

Signed-off-by: Julien Ponge <julien.ponge@insa-lyon.fr>
Merge pull request #183 from yloiseau/bugfix/NamedStructDeclaration

Make ASTStructDeclaration implements NamedNode