rpm/constraints.in: add static multibuild packages
    
    Commit 841012b049a5 (rpm/mkspec: use kernel-source: prefix for
    constraints on multibuild) added "kernel-source:" prefix to the
    dynamically generated kernels. But there are also static ones like
    kernel-docs. Those fail to build as the constraints are still not
    applied.
    
    So add the prefix also to the static ones.
    
    Note kernel-docs-rt is given kernel-source-rt prefix. I am not sure it
    will ever be multibuilt...