Remove BOM from gl.xml

This was inserted in #343, probably as an accidental side-effect of
someone's editor - it is not mentioned in the PR comments - and is
breaking some downstream scripts which consume the file.

xml/gl.xml

diff --git a/xml/gl.xml b/xml/gl.xml
index 06af90e..75755ba 100644
--- a/xml/gl.xml
+++ b/xml/gl.xml
@@ -1,4 +1,4 @@
-<?xml version="1.0" encoding="UTF-8"?>
+<?xml version="1.0" encoding="UTF-8"?>
 <registry>
     <comment>
 Copyright 2013-2020 The Khronos Group Inc.