cad/py-gmsh: fix build on powerpc64

/wrkdirs/usr/ports/cad/py-gmsh/work-py39/gmsh-4.11.1-source/contrib/eigen/Eigen/src/Core/arch/AltiVec/PacketMath.h:1345:32: error: use of undeclared identifier 'vec_sqrt'; did you mean 'vec_rsqrt'?

BF16_TO_F32_UNARY_OP_WRAPPER(vec_sqrt, a);
                             ^~~~~~~~
                             vec_rsqrt