rust: opp: Make the doctest example depend on CONFIG_OF

rust: opp: Make the doctest example depend on CONFIG_OF

The doctest example uses a function only available for CONFIG_OF and so
the build with doc tests fails when it isn't enabled.

  error[E0599]: no function or associated item named `from_of_cpumask`
  found for struct `rust_doctest_kernel_alloc_kbox_rs_4::kernel::opp::Table`
  in the current scope

Fix this by making the doctest depend on CONFIG_OF.

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202505260856.ZQWHW2xT-lkp@intel.com/
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Link: https://patch.msgid.link/a80bfedcb4d94531dc27d3b48062db5042078e88.1748237646.git.viresh.kumar@linaro.org
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>