lua: fix non-shared library build

Building without BR2_PACKAGE_LUA_SHARED_LIBRARY failed because of
missing -ldl.  dl is used by lua's dynamic module loading.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Acked-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>