C++ modernize: Pragma once (#6264)

* Migrate cpp headers to pragma once

src/util/base64.h

@@ -1,10 +1,26 @@
-#ifndef BASE64_HEADER
-#define BASE64_HEADER
+/*
+Minetest
+Copyright (C) 2013-2017 celeron55, Perttu Ahola <celeron55@gmail.com>
+
+This program is free software; you can redistribute it and/or modify
+it under the terms of the GNU Lesser General Public License as published by
+the Free Software Foundation; either version 2.1 of the License, or
+(at your option) any later version.
+
+This program is distributed in the hope that it will be useful,
+but WITHOUT ANY WARRANTY; without even the implied warranty of
+MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+GNU Lesser General Public License for more details.
+
+You should have received a copy of the GNU Lesser General Public License along
+with this program; if not, write to the Free Software Foundation, Inc.,
+51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+*/
+
+#pragma once
 
 #include <string>
 
 bool base64_is_valid(std::string const& s);
 std::string base64_encode(unsigned char const* , unsigned int len);
 std::string base64_decode(std::string const& s);
-
-#endif // BASE64_HEADER