Subzero: Fix the g++ build (e.g. Windows).

BUG= https://code.google.com/p/nativeclient/issues/detail?id=4006
R=kschimpf@google.com

Review URL: https://codereview.chromium.org/773583004
diff --git a/src/IceELFSection.cpp b/src/IceELFSection.cpp
index 1910e77..fd49e61 100644
--- a/src/IceELFSection.cpp
+++ b/src/IceELFSection.cpp
@@ -55,6 +55,7 @@
   else
     Unique = GlobalSymbols.insert(std::make_pair(Key, NewSymbol)).second;
   assert(Unique);
+  (void)Unique;
 }
 
 void ELFSymbolTableSection::noteUndefinedSym(const IceString &Name,