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);
           }
e='submit' value='switch'/></form></td></tr>
<tr><td class='sub'>Wojtek's customized Guix</td><td class='sub right'></td></tr></table>
<table class='tabs'><tr><td>
<a href='/guix/about/'>about</a><a href='/guix/'>summary</a><a href='/guix/refs/?id=11ed4108fbc665f6ff76263e602a8057a3da83ad'>refs</a><a class='active' href='/guix/log/gnu/packages/libftdi.scm'>log</a><a href='/guix/tree/gnu/packages/libftdi.scm?id=11ed4108fbc665f6ff76263e602a8057a3da83ad'>tree</a><a href='/guix/commit/gnu/packages/libftdi.scm?id=11ed4108fbc665f6ff76263e602a8057a3da83ad'>commit</a><a href='/guix/diff/gnu/packages/libftdi.scm?id=11ed4108fbc665f6ff76263e602a8057a3da83ad'>diff</a></td><td class='form'><form class='right' method='get' action='/guix/log/gnu/packages/libftdi.scm'>
<input type='hidden' name='id' value='11ed4108fbc665f6ff76263e602a8057a3da83ad'/><select name='qt'>
<option value='grep'>log msg</option>
<option value='author'>author</option>
<option value='committer'>committer</option>
<option value='range'>range</option>
</select>
<input class='txt' type='search' size='10' name='q' value=''/>
<input type='submit' value='search'/>
</form>
</td></tr></table>
<div class='path'>path: <a href='/guix/log/?id=11ed4108fbc665f6ff76263e602a8057a3da83ad'>root</a>/<a href='/guix/log/gnu?id=11ed4108fbc665f6ff76263e602a8057a3da83ad'>gnu</a>/<a href='/guix/log/gnu/packages?id=11ed4108fbc665f6ff76263e602a8057a3da83ad'>packages</a>/<a href='/guix/log/gnu/packages/libftdi.scm?id=11ed4108fbc665f6ff76263e602a8057a3da83ad'>libftdi.scm</a></div><div class='content'><table class='list nowrap'><tr class='nohover'><th class='left'>Age</th><th class='left'>Commit message (<a href='/guix/log/gnu/packages/libftdi.scm?id=11ed4108fbc665f6ff76263e602a8057a3da83ad&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2016-09-26 14:30:37 -0400'>2016-09-26</span></td><td><a href='/guix/commit/gnu/packages/libftdi.scm?id=58478cc1cb225e2eed979afa4021f850f3ebf121'>gnu: libftdi: Propagate the libusb input.</a><span class='msg-avail'>...</span></td><td>Danny Milosavljevic</td></tr>
<tr><td><span title='2014-08-05 20:00:21 -0400'>2014-08-05</span></td><td><a href='/guix/commit/gnu/packages/libftdi.scm?id=f0176a9f185f081535b02764b3bfe1151ce315be'>gnu: libftdi: Fix mistakes in package description.</a><span class='msg-avail'>...</span></td><td>Mark H Weaver</td></tr>
<tr><td><span title='2014-07-24 09:42:32 +0200'>2014-07-24</span></td><td><a href='/guix/commit/gnu/packages/libftdi.scm?id=77362186831b8c07e33b2e8ef0b4c09a446893a4'>gnu: Add libftdi.</a><span class='msg-avail'>...</span></td><td>Manolis Ragkousis</td></tr>