Older version of gcc mistakenly missed out on linking libdl without it.

Change-Id: I5c12972c140348fa504e4ff4acb81655b445b4a5
Reviewed-on: https://swiftshader-review.googlesource.com/c/24268Reviewed-by: Chris Forbes <chrisforbes@google.com>
Tested-by: Chris Forbes <chrisforbes@google.com>