kernel-source-rt from Kernel:SLE15-SP3-RT-LTSS project

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For pool run the following as root:
zypper addrepo https://download.opensuse.org/repositories/Kernel:SLE15-SP3-RT-LTSS/pool/Kernel:SLE15-SP3-RT-LTSS.repo
zypper refresh
zypper install kernel-source-rt