build: Don't import jemalloc (#15942)

py-polars/src/lib.rs

@@ -42,7 +42,7 @@ mod sql;
 mod to_numpy;
 mod utils;
 
-#[cfg(all(target_family = "unix", not(use_mimalloc)))]
+#[cfg(all(target_family = "unix", not(use_mimalloc), not(default_allocator)))]
 use jemallocator::Jemalloc;
 #[cfg(any(not(target_family = "unix"), use_mimalloc))]
 use mimalloc::MiMalloc;