Add missing copyright header in tools/build-mingw64.sh (#1727)

Author: jberdine

Diff for: tools/build-mingw64.sh

@@ -1,4 +1,14 @@
 #!/bin/bash
+##########################################################################
+#                                                                        #
+#                              OCamlFormat                               #
+#                                                                        #
+#            Copyright (c) Facebook, Inc. and its affiliates.            #
+#                                                                        #
+#      This source code is licensed under the MIT license found in       #
+#      the LICENSE file in the root directory of this source tree.       #
+#                                                                        #
+##########################################################################
 
 # Script to build `ocamlformat' under Windows, using the `mingw64' toolchain.
 # All it requires is a standard Cygwin installation with the `mingw64'