Fix SIMD mask accessors compilation with Emscripten

[jdumas]

Currently compiling embree with emscripten leads to the following error:

In file included from /home/runner/work/cpp_test/cpp_test/embree/common/simd/sse.h:31:
/home/runner/work/cpp_test/cpp_test/embree/common/simd/vboolf4_sse2.h:125:117: error: no member named 'm128i' in 'embree::vboolf_impl<4>'
  125 |   __forceinline vboolf4 operator ==(const vboolf4& a, const vboolf4& b) { return _mm_castsi128_ps(_mm_cmpeq_epi32(a.m128i(), b.m128i())); }
      |                                                                                                                   ~ ^
/home/runner/work/cpp_test/cpp_test/embree/common/simd/vboolf4_sse2.h:125:128: error: no member named 'm128i' in 'embree::vboolf_impl<4>'
  125 |   __forceinline vboolf4 operator ==(const vboolf4& a, const vboolf4& b) { return _mm_castsi128_ps(_mm_cmpeq_epi32(a.m128i(), b.m128i())); }
      |                                                                                                                              ~ ^
/home/runner/work/cpp_test/cpp_test/embree/common/simd/vboolf4_sse2.h:168:49: error: no member named 'm128i' in 'embree::vboolf_impl<4>'
  168 |     return _mm_castsi128_ps(_mm_shuffle_epi32(v.m128i(), _MM_SHUFFLE(i3, i2, i1, i0)));
      |                                               ~ ^
In file included from main.cpp:1:

This PR should fix it. For funsies here's a small LLM-generated repro with a GitHub Actions run showing the compile error. It might be worth adding Emscripten to the Embree CI workflow if you guys are interested (could do that as a separate PR).