Add License headers to cmake code

cmake/Findcmark.cmake

@@ -1,3 +1,6 @@
+# SPDX-FileCopyrightText: 2019 Black Hat <bhat@encom.eu.org>
+# SPDX-License-Identifier: GPL-3.0-only
+
 #
 # CMake module to search for the cmark library
 #

cmake/Flatpak.cmake

@@ -1,3 +1,6 @@
+# SPDX-FileCopyrightText: 2020 Carl Schwan <carl@carlschwan.eu>
+# SPDX-License-Identifier: BSD-2-Clause
+
 include(GNUInstallDirs)
 
 # Include FontConfig config which uses the Emoji One font from the