Add explicit instanciations of search_dispatch_implem (#3860)

Summary:
Since commit 32f0e8c (#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.

Pull Request resolved: #3860

Reviewed By: mengdilin

Differential Revision: D62762224

Pulled By: mnorris11

fbshipit-source-id: 938d5bc1917ef947847b1d8fcc3e40cea52e8367

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);