Don't export obj_dat.h.

This isn't a header file that makes sense to export; any compilation unit which
includes it will gain a bunch of static arrays.

Change-Id: Ic698b74bdf758506a53d4eba19ab8b0f49a11ef7
Reviewed-on: https://boringssl-review.googlesource.com/1692
Reviewed-by: Adam Langley <agl@google.com>