rgw/rgw_sal_motr: ignore clang warning

rgw/rgw_sal_motr: ignore clang warning

clang++-13 complains like:

/usr/include/motr/addb2/histogram.h:99:44: error: arithmetic between different enumeration types ('(unnamed enum at /usr/include/motr/addb2/internal.h:44:1)' and '(unnamed enum at /usr/include/motr/addb2/counter.h:49:1)') is deprecated [-Werror,-Wdeprecated-anon-enum-enum-conversion]
        M0_ADDB2_HIST_BUCKETS = 2 * (VALUE_MAX_NR - M0_ADDB2_COUNTER_VALS - 2)
                                     ~~~~~~~~~~~~ ^ ~~~~~~~~~~~~~~~~~~~~~

so let's silence it.

Signed-off-by: Kefu Chai <tchaikov@gmail.com>