Fatal error in ConsumeMessagesCommand.php if pcntl extension is missing

### Symfony version(s) affected

6.3.5

### Description

Executing `messenger:consume` fails with a fatal error `Undefined constant "SIGTERM"` if pcntl extension is not installed.

### How to reproduce

Run the command on a system with no pcntl extension.

### Possible Solution

Use `if defined('SIGTERM') ....` as in other parts of the code.

### Additional Context

_No response_