mesa3d: dragonboards: add format_srgb.c to prebuilts

While building Android for dragonboards, the build fails since it
can't find the generated file format_srgb.c.

To work around this problem, introduce prebuilt-intermediates for
format_srgb.c from the current source tree.

Change-Id: I4c96e040f6b7c633ccf7f3b34c742a7fb6ee6592
Signed-off-by: Sumit Semwal <sumit.semwal@linaro.org>