make_ctags: use -I option to ignore pg_node_attr macro

Hi,

I found that tag files generated by src/tools/make_ctags
doesn't include some keywords, that were field names of node
structs, for example norm_select in RestrictInfo. Such fields
are defined with pg_node_attr macro that was introduced
recently, like:

    Selectivity norm_selec pg_node_attr(equal_ignore);

In this case, pg_node_attr is mistakenly interpreted to be
the name of the field. So, I propose to use -I option of ctags
to ignore the marco name. Attached is a patch to do it.

Regards,
Yugo Nagata 

-- 
Yugo NAGATA <nagata@sraoss.co.jp>