Since commit 7f80a9ff

 ("vc4: Introduce XML-based packet header
generation like Intel's."), the vc4 build on Android is broken:

out/target/product/linaro_x86_64/gen/STATIC_LIBRARIES/libmesa_broadcom_genxml_intermediates/broadcom/cle/v3d_packet_v21_pack.h:12:10: fatal error: 'v3d_packet_helpers.h' file not found
external/mesa3d/src/gallium/drivers/vc4/vc4_cl_dump.c:28:10: fatal error: 'vc4_packet.h' file not found

The path of the generated header needs to be fixed since we build out of
tree.

Acked-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Rob Herring <robh@kernel.org>