[headache] fix configuration for .xml header generation

headers/headache_config.txt

@@ -171,7 +171,8 @@
 ########
 # XML #
 ########
-| ".*\.xml" -> frame open: "<!--" line: "-" close: "-->"
+| ".*\.xml" -> frame open: "<!--" line: " " close: "-->"
+| ".*\.xml" -> skip match: "<\?xml.*\?>"
 
 #######
 # DTD #

src/plugins/markdown-report/share/acsl.xml

-<!---------------------------------------------------------------------------->
+<?xml version="1.0" encoding="UTF-8"?>
+<!--                                                                        -->
 <!--                                                                        -->
 <!--  This file is part of Frama-C.                                         -->
 <!--                                                                        -->
@@ -18,9 +19,8 @@
 <!--  See the GNU Lesser General Public License version 2.1                 -->
 <!--  for more details (enclosed in the file licenses/LGPLv2.1).            -->
 <!--                                                                        -->
-<!---------------------------------------------------------------------------->
+<!--                                                                        -->
 
-<?xml version="1.0" encoding="UTF-8"?>
 <language name="ACSL" version="1" extensions="*.acsl"
           section="Sources" kateversion="2.4">
 <highlighting>