Use C++17 [[fallthrough]] in faiss/utils/distances_simd.cpp (facebookresearch#2913)

Summary: Pull Request resolved: facebookresearch#2913

Reviewed By: algoriddle

Differential Revision: D46603510

fbshipit-source-id: 374d530d79176ac553b40d5ad04bf83d4920b107

Author: r-barnes

faiss/utils/distances_simd.cpp

@@ -306,8 +306,10 @@ static inline __m128 masked_read(int d, const float* x) {
     switch (d) {
         case 3:
             buf[2] = x[2];
+            [[fallthrough]];
         case 2:
             buf[1] = x[1];
+            [[fallthrough]];
         case 1:
             buf[0] = x[0];
     }