This file gets included by AudioHRTF_avx512.cpp, which contains code
that should only be getting executed on CPUs with the appropriate support.
Unfortunately, when that file is compiled with -mavx512f, GCC also generates
AVX instructions in the code that initializes the value of SQUARE_ROOT_OF_2,
and this crashes on CPUs without AVX because it runs unconditionally.
Avoid the issue entirely by just making it a constant so no code needs to
be run.