ecpg: use our instead of my in parse.pl to fix perlcritic complaint

In db0a272d123 I used open(our $something, ...), which perlcritic doesn't
like. It looks like the warning is due to perlcritic knowing about 'my' but
not 'our' when checking for bareword file handles.

However, it's clearly unnecessary to use "our" here, change it to "my".

Via buildfarm member crake and discussion with Tom Lane.

Discussion: https://postgr.es/m/20220718215042.sl3hivoupdb7lkwv@awork3.anarazel.de

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