Ubuntu: SAUCE: IMX51: export symbol of ipu_clear_buffer_ready function

Without exporting the symbol of ipu_clear_buffer_ready, linking driver
drivers/media/video/mxc/output/mxc_v4l2_output.c will fail because the function
ipu_clear_buffer_ready is undefined.

This patch is necessary for building latest Freescale iMX51 BSP and will be
submitted for Freescale to integrated into next BSP release.

Signed-off-by: Bryan Wu <bryan.wu@canonical.com>