Fix alignment of select fields

[eduardomozart]

Checklist before requesting a review

Please delete options that are not relevant.

• I have read the CONTRIBUTING document.
• I have performed a self-review of my code.
• I have added tests that prove my fix is effective or that my feature works.
• This change requires a documentation update.

Description

• It fixes # (issue number, if applicable)
• Here is a brief description of what this PR does

Screenshots (if appropriate):

The select dropdown has been limited to a max width of 270px, but it's container not. It creates a situation where the select has it width limited but not it's container, which makes the buttons (like "i" [info] and "+" [toadd]) not aligned with the fields.

After applying this PR it's aligned as expected:

This behavior can be seen in Agent task properties page of GLPI Inventory plugin (https://glpi.example.com/plugins/glpiinventory/front/task.form.php?id=1) which uses a fixed width to their selects as seen in https://github.com/glpi-project/glpi-inventory-plugin/blob/c17dbab0b5ed6c8ae8bfb885ca44a40de47e0f34/inc/commonview.class.php#L193-L200