fix warnings for building on win32

Change-Id: If6e11ba3d681e831d7d98662c0abdd2ac16b3811
diff --git a/vp8/vp8_cx_iface.c b/vp8/vp8_cx_iface.c
index 920c763..8509b9e 100644
--- a/vp8/vp8_cx_iface.c
+++ b/vp8/vp8_cx_iface.c
@@ -227,7 +227,7 @@
 
     if (cfg->ts_number_layers > 1)
     {
-        int i;
+        unsigned int i;
         RANGE_CHECK_HI(cfg, ts_periodicity, 16);
 
         for (i=1; i<cfg->ts_number_layers; i++)