u_math.h: fix 64 to 32 bit truncation warning

Signed-off-by: Alon Levy <alevy@redhat.com>
Reviewed-by: Brian Paul <brianp@vmware.com>
diff --git a/src/gallium/auxiliary/util/u_math.h b/src/gallium/auxiliary/util/u_math.h
index f95c111..0113fb1 100644
--- a/src/gallium/auxiliary/util/u_math.h
+++ b/src/gallium/auxiliary/util/u_math.h
@@ -799,7 +799,7 @@
 #if defined(HAVE___BUILTIN_BSWAP64)
    return __builtin_bswap64(n);
 #else
-   return ((uint64_t)util_bswap32(n) << 32) |
+   return ((uint64_t)util_bswap32((uint32_t)n) << 32) |
           util_bswap32((n >> 32));
 #endif
 }