Add explicit instanciations of search_dispatch_implem

Since commit 32f0e8c (facebookresearch#3190) faiss fails to build on gcc7.

For some reason gcc7 needs these explicit instanciations (or rather, for some reason later versions are OK without them).
This commit partially revers that commit, adding back the explicit instanciations.

faiss/IndexFastScan.cpp

@@ -549,6 +549,22 @@ void IndexFastScan::search_implem_14(
     }
 }
 
+template void IndexFastScan::search_dispatch_implem<true>(
+        idx_t n,
+        const float* x,
+        idx_t k,
+        float* distances,
+        idx_t* labels,
+        const NormTableScaler* scaler) const;
+
+template void IndexFastScan::search_dispatch_implem<false>(
+        idx_t n,
+        const float* x,
+        idx_t k,
+        float* distances,
+        idx_t* labels,
+        const NormTableScaler* scaler) const;
+
 void IndexFastScan::reconstruct(idx_t key, float* recons) const {
     std::vector<uint8_t> code(code_size, 0);
     BitstringWriter bsw(code.data(), code_size);