databases/mysql81-server: fix build on powerpc64*

/wrkdirs/usr/ports/databases/mysql81-server/work/mysql-8.1.0/boost/boost_1_77_0/boost/mpl/aux_/integral_wrapper.hpp:73:31: error: integer value -1 is outside the valid range of values [0, 3] for the enumeration type 'int_float_mixture_enum' [-Wenum-constexpr-conversion]
/wrkdirs/usr/ports/databases/mysql81-server/work/mysql-8.1.0/boost/boost_1_77_0/boost/mpl/aux_/static_cast.hpp:24:47: note: expanded from macro 'BOOST_MPL_AUX_STATIC_CAST'

24 | #   define BOOST_MPL_AUX_STATIC_CAST(T, expr) static_cast<T>(expr)