multimedia/qt6-multimedia: fix build on powerpc64

In file included from /wrkdirs/usr/ports/multimedia/qt6-multimedia/work/qtmultimedia-everywhere-src-6.4.1/src/resonance-audio/../3rdparty/eigen/Eigen/Core:210:
/wrkdirs/usr/ports/multimedia/qt6-multimedia/work/qtmultimedia-everywhere-src-6.4.1/src/resonance-audio/../3rdparty/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
/wrkdirs/usr/ports/multimedia/qt6-multimedia/work/qtmultimedia-everywhere-src-6.4.1/src/resonance-audio/../3rdparty/eigen/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);\
                     ^
/usr/lib/clang/13.0.0/include/altivec.h:8472:34: note: 'vec_rsqrt' declared here
static vector float __ATTRS_o_ai vec_rsqrt(vector float __a) {