Build liblua as a group, not as a static lib.

This prevents a number of linking warnings about the fact that liblua.a
has no symbols.

Change-Id: I9ee7d8dab92648928ba0fb142d29486d2723ad72
diff --git a/BUILD.gn b/BUILD.gn
index c600c92..dcbac98 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -45,7 +45,7 @@
   ]
 }
 
-static_library("liblua") {
+group("liblua") {
   deps = [
     ":lua_core",
     ":lua_lib",