Fix signature obtaining in rpm_info probe

[Xeicker]

Description

Fix rpminfo probe to correctly obtain signature_keyid in OL9, and possibly RHEL9

Testing

With existing maint1.3 code the key isn't obtained in OL9:

## Executing any rule scan with --oval-results argument
$ ./oscap_wrapper xccdf eval --profile stig --rule xccdf_org.ssgproject.content_rule_set_password_hashing_min_rounds_logindefs --oval-results /usr/share/xml/scap/ssg/content/ssg-ol9-ds.xml
## Check the cpe oval results
$ grep signature_keyid ssg-ol9-cpe-oval.xml.result.xml 
            <lin-sys:signature_keyid>0</lin-sys:signature_keyid>

Applying this change this information is correctly filled:

$ grep signature_keyid ssg-ol9-cpe-oval.xml.result.xml 
            <lin-sys:signature_keyid>bc4d06a08d8b756f</lin-sys:signature_keyid>

[jan-cerny]

This works for me on Fedora as well.