i965: make swizzle_for_size() return unsigned

Silences a warning about comparing to an unsigned variable.  It looks like
the result of swizzle_for_size() is always assigned to unsigned vars.

Reviewed-by: Chad Versace <chad@chad-versace.us>

src/mesa/drivers/dri/i965/brw_vec4.h

@@ -47,10 +47,10 @@ class dst_reg;
  * channels, as that will tell optimization passes that those other
  * channels are used.
  */
-static int
+static unsigned
 swizzle_for_size(int size)
 {
-   static const int size_swizzles[4] = {
+   static const unsigned size_swizzles[4] = {
       BRW_SWIZZLE4(SWIZZLE_X, SWIZZLE_X, SWIZZLE_X, SWIZZLE_X),
       BRW_SWIZZLE4(SWIZZLE_X, SWIZZLE_Y, SWIZZLE_Y, SWIZZLE_Y),
       BRW_SWIZZLE4(SWIZZLE_X, SWIZZLE_Y, SWIZZLE_Z, SWIZZLE_Z),