gio: Add some missing license and copyright headers

These headers have all been written manually, by looking through the git
log for each file and noting the copyright of each significant
contribution.

Signed-off-by: Philip Withnall <pwithnall@endlessos.org>

Helps: #1415

gio/gwin32api-storage.h

@@ -1,3 +1,11 @@
+/*
+ * SPDX-FileCopyrightText: This file has no copyright assigned and is placed in the Public Domain.
+ * SPDX-License-Identifier: CC0-1.0
+ *
+ * This file is part of the w64 mingw-runtime package.
+ * No warranty is given; refer to https://github.com/kinke/mingw-w64-crt/blob/master/DISCLAIMER.PD.
+ */
+
 struct DateTime;
 
 typedef struct DateTime {