libre: linux-libre-64: don't try to ship non-existing headers
    
    Without that fix, when building for Parabola armv7h we have:
     |  ==> Starting package_linux-libre-64-headers()...
     |  Installing build files...
     |  Installing headers...
     |  cp: cannot stat 'arch/arm64/mach-dove/include': No such file or directory
     |  ==> ERROR: A failure occurred in package_linux-libre-64-headers().
     |      Aborting...
    
    However these headers do exist for armv7h. As in this case we are
    building an aarch64 kernel, we don't need armv7h headers anyway.
    
    Signed-off-by: Denis 'GNUtoo' Carikli <GNUtoo@cyberdimension.org>