ddraw: Sign-compare warning fix

[Andrew Talbot]

Changelog:
    ddraw: Sign-compare warning fix.

diff --git a/dlls/ddraw/surface.c b/dlls/ddraw/surface.c
index fc481e4..727bcce 100644
--- a/dlls/ddraw/surface.c
+++ b/dlls/ddraw/surface.c
@@ -285,7 +285,7 @@ IDirectDrawSurfaceImpl_Release(IDirectDrawSurface7 *iface)
         IDirectDrawSurfaceImpl *surf;
         IDirectDrawImpl *ddraw;
         IUnknown *ifaceToRelease = This->ifaceToRelease;
-        int i;
+        UINT i;
 
         /* Complex attached surfaces are destroyed implicitly when the root is released */
         EnterCriticalSection(&ddraw_cs);