fix missing exported header for tests

Author: SamGondelman

extension/data_loader/targets.bzl

@@ -72,10 +72,11 @@ def define_common_targets():
         srcs = [
             "mmap_data_loader.cpp"
         ] + (["mman_windows.cpp"] if host_info().os.is_windows else []),
-        headers = [
+        headers = ["mman_windows.h"] if host_info().os.is_windows else [],
+        exported_headers = [
             "mman.h",
-        ] + (["mman_windows.h"] if host_info().os.is_windows else []),
-        exported_headers = ["mmap_data_loader.h"],
+            "mmap_data_loader.h"
+        ],
         visibility = [
             "//executorch/test/...",
             "//executorch/extension/pybindings/...",