Fix gcc error for static_cast the parameters of DCHECK_GT macro to unsigned in list.h.

src/list.h

Index: src/list.h
diff --git a/src/list.h b/src/list.h
index 60862e8d0289ee8ed4a6e6abe7960553032b8ffb..b59ece463efba2c64eb0ad7a1e763ef8052c9519 100644
--- a/src/list.h
+++ b/src/list.h
@@ -65,7 +65,7 @@ class List {
   // backing store (e.g. Add).
   inline T& operator[](int i) const {
     DCHECK_LE(0, i);
-    DCHECK_GT(length_, i);
+    DCHECK_GT(static_cast<unsigned>(length_), static_cast<unsigned>(i));
     return data_[i];
   }
   inline T& at(int i) const { return operator[](i); }