Fix LLVM-19 compilation issue in faiss/AutoTune.cpp (#4220)

Summary:
Pull Request resolved: #4220

LLVM-19 is incoming. This fixes an issue preventing it. Delays to previous platform upgrades cost $3M/week.

Reviewed By: dtolnay

Differential Revision: D70449926

fbshipit-source-id: 20e0882b9363670d6c010e1c7870cb04155a3a9d

faiss/AutoTune.cpp

@@ -314,7 +314,7 @@ bool ParameterSpace::combination_ge(size_t c1, size_t c2) const {
 }
 
 #define DC(classname) \
-    const classname* ix = dynamic_cast<const classname*>(index)
+    [[maybe_unused]] const classname* ix = dynamic_cast<const classname*>(index)
 
 static void init_pq_ParameterRange(
         const ProductQuantizer& pq,