test: fix boostmutiprec test to compile with older Boost versions

test: fix boostmutiprec test to compile with older Boost versions

Eigen boostmultiprec test redefines a symbol that is already defined inside Boot Math [1]. Boost has fixed it recently [2], but this patch avoids errors if Boost version was less than 1.77.

https://github.com/boostorg/math/blob/boost-1.76.0/include/boost/math/policies/policy.hpp#L18 https://github.com/boostorg/math/commit/68307123029676ba5cb316f8dd1d1c98d1fc7b23#diff-c7a8e5911c2e6be4138e1a966d762200f147792ac16ad96fdcc724313d11f839