Incorrect log level assignment for auditd messages without PRIORITY field

# Environment

- dlt-daemon: (v2.18.10: 0f2d4cfffada6f8448a2cb27995b38eb4271044f )
- systemd: 244.5
- audit: 2.8.5
- linux: 5.4

# Issue

Currently, dlt-daemon assigns the log level "Emergency" to all journal messages from auditd that lack a PRIORITY field.
This behavior originates from the current implementation of [get_journal_msg()](https://github.com/COVESA/dlt-daemon/blob/master/src/system/dlt-system-journal.c#L202-L256), which does not handle cases where the PRIORITY field is missing.

## Example message from auditd
```
Tue 2024-12-24 17:20:49.383000 UTC+9 [s=0272031534e84e679782433f87f4562a;i=56a;b=a5f717e39b1644779bbdff5c09e85249;m=fcc71;t=629ffca66ae5a;x=6093bf34f8a4fa5d]
_BOOT_ID=a5f717e39b1644779bbdff5c09e85249
_MACHINE_ID=076f208e43f94dab8f210263f82252d0
_HOSTNAME=mymachine
_UID=0
_TRANSPORT=audit
_AUDIT_TYPE=1130
SYSLOG_FACILITY=4
SYSLOG_IDENTIFIER=audit
_AUDIT_TYPE_NAME=SERVICE_START
_PID=1
_AUDIT_LOGINUID=4294967295
_AUDIT_SESSION=4294967295
_SELINUX_CONTEXT=system_u:system_r:init_t:s0
AUDIT_FIELD_COMM=systemd
AUDIT_FIELD_EXE=/lib/systemd/systemd
AUDIT_FIELD_HOSTNAME=?
AUDIT_FIELD_ADDR=?
AUDIT_FIELD_TERMINAL=?
AUDIT_FIELD_RES=success
_SOURCE_REALTIME_TIMESTAMP=1735028449383000
_AUDIT_ID=17
MESSAGE=SERVICE_START pid=1 uid=0 auid=4294967295 ses=4294967295 subj=system_u:system_r:init_t:s0 msg='unit=systemd-udevd comm="systemd" exe="/lib/systemd/systemd" hostname=? addr=? terminal=? res=success'
AUDIT_FIELD_UNIT=systemd-udevd
```

## Expected Behavior

Since messages from auditd with SYSLOG_FACILITY=4 do not contain a PRIORITY field, dlt-daemon should determine the appropriate log level based on other fields such as _AUDIT_TYPE.

## Possible Improvement

- Modify get_journal_msg() to:
- Check SYSLOG_FACILITY and _AUDIT_TYPE when PRIORITY is missing.
- Use a fallback mechanism to infer a more accurate log level rather than defaulting to "Emergency".
- e.g. If _AUDIT_TYPE is `AVC`, messages will be classified to "Error" and if not, messages will be classified to "INFO".

Would appreciate any thoughts or suggestions on how best to handle this!