configure.in:  silence warning about libxenomai useless dependency

`libxenomai` symbols aren't directly used in some cases, but not all;
silence this warning during Debian package builds:

    dpkg-shlibdeps: warning: package could avoid a useless dependency
    if debian/machinekit-xenomai/usr/lib/linuxcnc/xenomai/rtapi.so
    debian/machinekit-xenomai/usr/lib/linuxcnc/ulapi-xenomai.so were
    not linked against libxenomai.so.0 (they use none of the library's
    symbols)