Fix a string comparison in C++ 11.

Taken from Debian:
https://sources.debian.org/src/dbus-c++/0.9.0-8.1/debian/patches/06_fix_gcc-7_ftbfs.patch/

--- a/src/pipe.cpp
+++ b/src/pipe.cpp
@@ -83,5 +83,5 @@ ssize_t Pipe::read(void *buffer, unsigne
 void Pipe::signal()
 {
   // TODO: ignoring return of read/write generates warning; maybe relevant for eventloop work...
-  ::write(_fd_write, '\0', 1);
+  ::write(_fd_write, "\0", 1);
 }
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<div class='path'>path: <a href='/guix/log/?id=35466639a0cc4450303826f8483318c0e24719d0'>root</a>/<a href='/guix/log/.patman?id=35466639a0cc4450303826f8483318c0e24719d0'>.patman</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/.patman?id=35466639a0cc4450303826f8483318c0e24719d0&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2023-05-01 12:39:11 -0400'>2023-05-01</span></td><td><a href='/guix/commit/.patman?id=f21007ce4aceeacf5bbfc7cd45d526073141f194'>.patman: Remove get_maintainer_script option.</a><span class='msg-avail'>...<span class='msg-tooltip'>This is now handled at the level of git via its new '--header-cmd' option,
which produces X-Debbugs-CC headers.

* .patman (get_maintainer_script): Delete option.
</span></span></td><td>Maxim Cournoyer</td></tr>
<tr><td><span title='2023-02-25 23:10:59 -0500'>2023-02-25</span></td><td><a href='/guix/commit/.patman?id=cb0d8100b288b5b0d130820207db17764b7d2140'>.patman: Ignore bad tags.</a><span class='msg-avail'>...<span class='msg-tooltip'>The feature is not used in Guix, and it produces extraneous warnings.

* .patman (ignore_bad_tags): Set option to True.
</span></span></td><td>Maxim Cournoyer</td></tr>
<tr><td><span title='2022-12-28 15:36:33 -0500'>2022-12-28</span></td><td><a href='/guix/commit/.patman?id=fde5af3962c8fafc5d20e5d742cc7aea907f3563'>.patman: New configuration file.</a><span class='msg-avail'>...<span class='msg-tooltip'>* .patman: New file.

Series-to: 60218@debbugs.gnu.org
Series-cc: rekado@elephly.net, othacehe@gnu.org
Series-version: 2
</span></span></td><td>Maxim Cournoyer</td></tr>