* fixed: GC wrongly marks classes if Struct is not included into the build

[dkimitsa]

org_robovm_rt_bro_Struct might be not initialized if class is not present (e.g. in pure console app), in this case org_robovm_rt_bro_Struct will be null and GC will recognize generic classes as Structs and apply wrong GC Mark method

code that initialise it:

    org_robovm_rt_bro_Struct = rvmFindClassUsingLoader(env, "org/robovm/rt/bro/Struct", NULL);
    if (!org_robovm_rt_bro_Struct) {
        // We don't need Struct if it hasn't been compiled in
        rvmExceptionClear(env);
    } else {
        org_robovm_rt_bro_Struct_handle = rvmGetInstanceField(env, org_robovm_rt_bro_Struct, "handle", "J");
        if (!org_robovm_rt_bro_Struct_handle) return FALSE;
    }