Added -Wsign-compare.

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@7517 a5019735-40e9-0310-863c-91ae7b9d1cf9
diff --git a/exp-drd/drd_thread.c b/exp-drd/drd_thread.c
index 031a02f..b233199 100644
--- a/exp-drd/drd_thread.c
+++ b/exp-drd/drd_thread.c
@@ -543,7 +543,7 @@
  */
 static void thread_compute_minimum_vc(VectorClock* vc)
 {
-   int i;
+   unsigned i;
    Bool first;
    Segment* latest_sg;
 
@@ -564,7 +564,7 @@
 
 static void thread_compute_maximum_vc(VectorClock* vc)
 {
-   int i;
+   unsigned i;
    Bool first;
    Segment* latest_sg;
 
@@ -590,8 +590,8 @@
  */
 static void thread_discard_ordered_segments(void)
 {
+   unsigned i;
    VectorClock thread_vc_min;
-   int i;
 
    s_discard_ordered_segments_count++;