Fix warning about duplicate id in docbook XML

../newlib/libc/libc.xml:22242: element refentry: validity error : ID iconv already defined
    <refentry id="iconv">

Use a separate namespace for chaper ids, to avoid collision between the
ids for the chapter and function 'iconv', now that iconv documentation
is generated unconditionally.

newlib/doc/chapter-texi2docbook.py

@@ -29,7 +29,7 @@ def main():
             l = l.strip()
             l = l.lower()
             if first_node:
-                print ('<chapter id="%s" xmlns:xi="http://www.w3.org/2001/XInclude">' % l.replace(' ', '_'))
+                print ('<chapter id="%s_chapter" xmlns:xi="http://www.w3.org/2001/XInclude">' % l.replace(' ', '_'))
                 first_node = False
         elif l.startswith("@chapter "):
             l = l.replace("@chapter ", "", 1)