rtai-config-multi-kernel.patch:  suppress error messages

RTAI package build uses fake Linux sources; no need to expose these
errors

    checking linux utsname... x86_64-linux-gnu-gcc: error:
        [...]/debian/linux/include/linux/version.h: No such file or directory
    x86_64-linux-gnu-gcc: fatal error: no input files
    compilation terminated.
    not found