Fix build for gcc-4.2 opt/fast

Even though the code is safe, compiler flags a warning here, which are treated as errors for fast/opt. I know it's redundant but it has no side effects and fixes the compile.

src/mem/cache/prefetch/base.cc

@@ -138,7 +138,7 @@ BasePrefetcher::getPacket()
         return NULL;
     }
 
-    PacketPtr pkt;
+    PacketPtr pkt = *pf.begin();
     while (!pf.empty()) {
         pkt = *pf.begin();
         pf.pop_front();