[16.0][FIX] pos_membership_extension: add dependency

[flaenen]

Without the pos_membership module, it is possible to sell a membership without creating an invoice and without selecting a customer.

[OCA-git-bot]

Hi @legalsylvain, some modules you are maintaining are being modified, check this out!

[github-actions[bot]]

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[polchampion]

@legalsylvain what do you think?

[legalsylvain]

Hi. I don't understand why changing this dependency.

Maybe better put pos_membership as auto_install=True, if we thing that this module should be allways installed (if pos and membership are installed).

What do you think ?