[lld-link] Fix -Wunused-but-set-variable in -DLLVM_ENABLE_ASSERTIONS=off build. NFC

lld/COFF/Writer.cpp

@@ -595,6 +595,7 @@ void Writer::finalizeAddresses() {
     // If the verification above thought we needed thunks, we should have
     // added some.
     assert(addressesChanged);
+    (void)addressesChanged;
 
     // Recalculate the layout for the whole image (and verify the ranges at
     // the start of the next round).