math/gismo: fix build on powerpc64

/wrkdirs/usr/ports/math/gismo/work/gismo-21.12.0-882-g236b1b29/external/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

/wrkdirs/usr/ports/math/gismo/work/gismo-21.12.0-882-g236b1b29/external/Eigen/src/Core/arch/AltiVec/PacketMath.h:1302:22: note: expanded from macro 'BF16_TO_F32_UNARY_OP_WRAPPER'

Packet4f op_even = OP(a_even);\
                   ^