Factor.

windows/BUILD

@@ -0,0 +1,16 @@
+# Copyright 2023 Bradley D. Nelson
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+Importation('gen/windows_boot.h', '$src/windows/windows_boot.fs', header_mode='win', name='boot')
+Importation('gen/windows_boot_extra.h', '$src/windows/windows_boot_extra.fs', header_mode='win', name='boot')