wined3d: Fix a few sign compare warnings.
diff --git a/dlls/wined3d/wined3d_private.h b/dlls/wined3d/wined3d_private.h
index c90a77d..87947ee 100644
--- a/dlls/wined3d/wined3d_private.h
+++ b/dlls/wined3d/wined3d_private.h
@@ -2424,7 +2424,7 @@
 
     /* Some information about the shader behavior */
     struct stb_const_desc       bumpenvmatconst[MAX_TEXTURES];
-    char                        numbumpenvmatconsts;
+    unsigned char               numbumpenvmatconsts;
     struct stb_const_desc       luminanceconst[MAX_TEXTURES];
     char                        vpos_uniform;