swscale: Fix stack alignment for SSE

Although gcc guarantees 16 byte stack alignment, threads under WinXP
don't appear to be guaranteed to start stack aligned.  So fix the
alignment.

Signed-off-by: Ronald S. Bultje <rsbultje@gmail.com>

libswscale/swscale_unscaled.c

@@ -806,7 +806,7 @@ static int check_image_pointers(uint8_t *data[4], enum PixelFormat pix_fmt,
  * swscale wrapper, so we don't need to export the SwsContext.
  * Assumes planar YUV to be in YUV order instead of YVU.
  */
-int sws_scale(struct SwsContext *c, const uint8_t* const srcSlice[],
+int attribute_align_arg sws_scale(struct SwsContext *c, const uint8_t* const srcSlice[],
               const int srcStride[], int srcSliceY, int srcSliceH,
               uint8_t* const dst[], const int dstStride[])
 {