kernel.spec: Fix build dep on libelf-devel
    
    The dependency on libelf-devel is wrapped in "%if %{with_perf}".
    However, if you're building without perf (as fast-build.sh does) then
    you don't have this installed and the build will complain:
    
    Makefile:1037: "Cannot use CONFIG_STACK_VALIDATION, please install
    libelf-dev, libelf-devel or elfutils-libelf-devel"
    
    This just lifts the dependency out of the %if section.