[minor] Add fitContent width to DxcSelect

[PelayoFelgueroso]

Checklist
(Check off all the items before submitting)

• Build process is done without errors. All tests pass in the /lib directory.
• Self-reviewed the code before submitting.
• Meets accessibility standards.
• Added/updated documentation to /website as needed.
• Added/updated tests as needed.

Purpose
Add add fitContent width to the DxcSelect in order to implement the message input