Lua: Fix void return for non-void functions

Fixes issue #93

Commit #c3f3880d caused the functions
SWIGINTERN int SWIG_Lua_add_namespace_details(lua_State* L,
swig_lua_namespace* ns)
and
SWIGINTERN int SWIG_Lua_namespace_register(lua_State* L,
swig_lua_namespace* ns)
to return void when int returns were expected resulting in the build
failures for plplot's lua bindings for example. This commit fixes the
issue.