CMake: Don't cache LUA_LIBRARIES.

Don't cache the LUA_LIBRARIES variable. This matches the behavior of the
other library modules and fixes a compilation problem on my machine
where /usr/local/lib/liblua.dylib wasn't showing up in the various
build.make and link.txt files.

Change-Id: Ib75ef303f2e67b266a246621718d0ea2ab885dca
Reviewed-on: https://code.wireshark.org/review/5603
Reviewed-by: Gerald Combs <gerald@wireshark.org>