GCC14/C++23 compatibility fix

[alexge50]

Fixes the following error, when compiling with GCC 14, using -std=c++23:

<>/basisu/encoder/basisu_kernels_sse.cpp:41:
<>/basisu/encoder/cppspmd_sse.h: In member function ‘cppspmd_sse41::spmd_kernel::vfloat& cppspmd_sse41::spmd_kernel::store(vfloat&&, const vfloat&)’:
<>/basisu/encoder/cppspmd_sse.h:496:24: error: cannot bind non-const lvalue reference of type ‘cppspmd_sse41::spmd_kernel::vfloat&’ to an rvalue of type ‘cppspmd_sse41::spmd_kernel::vfloat’
  496 |                 return dst;
      |                        ^~~
<>/basisu/encoder/cppspmd_sse.h: In member function ‘cppspmd_sse41::spmd_kernel::vfloat& cppspmd_sse41::spmd_kernel::store_all(vfloat&&, const vfloat&)’:
<>/basisu/encoder/cppspmd_sse.h:508:24: error: cannot bind non-const lvalue reference of type ‘cppspmd_sse41::spmd_kernel::vfloat&’ to an rvalue of type ‘cppspmd_sse41::spmd_kernel::vfloat’
  508 |                 return dst;
      |                        ^~~

[CLAassistant]

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[MarkCallow]

Thank you for the fix.