fix src/headers brackets

Author: SamGondelman

extension/data_loader/targets.bzl

@@ -71,11 +71,11 @@ def define_common_targets():
         name = "mmap_data_loader",
         srcs = [
             "mmap_data_loader.cpp"
-        ] + ["mman_windows.cpp"] if host_info().os.is_windows else [],
-        exported_headers = [
+        ] + (["mman_windows.cpp"] if host_info().os.is_windows else []),
+        headers = [
             "mman.h",
-            "mmap_data_loader.h",
-        ] + ["mman_windows.h"] if host_info().os.is_windows else [],
+        ] + (["mman_windows.h"] if host_info().os.is_windows else []),
+        exported_headers = ["mmap_data_loader.h"],
         visibility = [
             "//executorch/test/...",
             "//executorch/extension/pybindings/...",