ast: Updated AST_NUM_OPERATORS for coherence with ast_operators

ast: Updated AST_NUM_OPERATORS for coherence with ast_operators

AST_NUM_OPERATORS stores the dimension of the ast_operators
enumeration but was not updated after its last modification.

This doesn't add any real modification for any code paths but it makes
sense for coherence.

v2 (Eric Engestrom): Just place the define at the end of the
                     enumeration, not below.

Signed-off-by: Andres Gomez <agomez@igalia.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Eric Engestrom <eric.engestrom@imgtec.com>