gate posix include with feature flag

Signed-off-by: Ava Hahn <ava@sunnypup.io>

src/sym.rs

@@ -18,6 +18,7 @@
 use crate::eval::eval;
 use crate::error::{Traceback, start_trace};
 use crate::segment::{Ctr, Seg, Type};
+#[cfg(feature="implicit-load")]
 use crate::stl::posix::POSIX_LOAD_NAME;
 use std::collections::HashMap;
 use std::fmt;