Fix a regression in 6.8 where the 'single' headings option was not recognized.

Taken from upstream:

  https://git.savannah.gnu.org/cgit/texinfo.git/commit/?id=13a8894fe2faa45b04033d7122a8fe7939ce6aa2

diff --git a/tp/Texinfo/XS/parsetexi/end_line.c b/tp/Texinfo/XS/parsetexi/end_line.c
index 4556780052..3fc2065616 100644
--- a/tp/Texinfo/XS/parsetexi/end_line.c
+++ b/tp/Texinfo/XS/parsetexi/end_line.c
@@ -778,8 +778,8 @@ kdbinputstyle_invalid:
     case CM_headings:
       {
         if (!strcmp (line, "off") || !strcmp (line, "on")
-            || !strcmp (line, "double") || !strcmp (line, "singleafter")
-            || !strcmp (line, "doubleafter"))
+            || !strcmp (line, "single") || !strcmp (line, "double")
+            || !strcmp (line, "singleafter") || !strcmp (line, "doubleafter"))
           {
             ADD_ARG(line);
           }
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