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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<tr><td><span title='2020-10-20 22:39:43 +0100'>2020-10-20</span></td><td><a href='/guix/commit/gnu/tests/databases.scm?id=bdcf4d88d58798eca7811c8b1fbd4638168d05c3'>services: databases: Don't specify a default postgresql version.</a><span class='msg-avail'>...</span></td><td>Christopher Baines</td></tr>
<tr><td><span title='2018-10-18 01:12:22 +0200'>2018-10-18</span></td><td><a href='/guix/commit/gnu/tests/databases.scm?id=39d7fdce453b0ca23ecbed72048647debbaa58a6'>services: dhcp-client: Deprecate 'dhcp-client-service' procedure.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2018-03-10 16:20:55 +0000'>2018-03-10</span></td><td><a href='/guix/commit/gnu/tests/databases.scm?id=69f7cf2bf4ca3ab792438a9e7fb46f78ef0f227a'>tests: databases: Add a system test for PostgreSQL.</a><span class='msg-avail'>...</span></td><td>Christopher Baines</td></tr>
<tr><td><span title='2017-10-08 17:31:49 +0100'>2017-10-08</span></td><td><a href='/guix/commit/gnu/tests/databases.scm?id=02bc41c4cedd15c6a76ecd5218f9b5848de8baea'>tests: databases: Add a simple test for MySQL.</a><span class='msg-avail'>...</span></td><td>Christopher Baines</td></tr>
<tr><td><span title='2017-10-06 21:24:30 +0100'>2017-10-06</span></td><td><a href='/guix/commit/gnu/tests/databases.scm?id=5266ff719e274056cb3e2b9740183f0063177255'>services: Add MongoDB.</a><span class='msg-avail'>...</span></td><td>Christopher Baines</td></tr>
<tr><td><span title='2017-08-15 22:24:15 +0100'>2017-08-15</span></td><td><a href='/guix/commit/gnu/tests/databases.scm?id=6230e155afd8c43c12ee3f03032aac34433db11a'>gnu: Fix memcached service startup.</a><span class='msg-avail'>...</span></td><td>Christopher Baines</td></tr>
<tr><td><span title='2017-07-30 09:08:30 +0100'>2017-07-30</span></td><td><a href='/guix/commit/gnu/tests/databases.scm?id=119fdd0d0e33492ef2b563295fe9564258d51401'>services: Add memcached.</a><span class='msg-avail'>...</span></td><td>Christopher Baines</td></tr>