Fix build failure with glibc 2.26.

Patch copied from upstream source repository:
https://sourceware.org/git/?p=valgrind.git;a=commit;h=2b5eab6a8db1b0487a3ad7fc4e7eeda6d3513626

diff --git a/memcheck/tests/linux/stack_changes.c b/memcheck/tests/linux/stack_changes.c
index a978fc2..7f97b90 100644
--- a/memcheck/tests/linux/stack_changes.c
+++ b/memcheck/tests/linux/stack_changes.c
@@ -10,7 +10,7 @@
 // This test is checking the libc context calls (setcontext, etc.) and
 // checks that Valgrind notices their stack changes properly.
 
-typedef  struct ucontext  mycontext;
+typedef  ucontext_t  mycontext;
 
 mycontext ctx1, ctx2, oldc;
 int count;
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<tr><td><span title='2016-11-06 16:17:16 +0000'>2016-11-06</span></td><td><a href='/guix/commit/gnu/packages/patches/lua-liblua-so.patch?id=6dbf1fec0d56152a9a893045cd508d337349e02a'>gnu: lua: Update to 5.3.3.</a><span class='msg-avail'>...</span></td><td>Marius Bakke</td></tr>
Update URL.
[home-page]: Update URL.


</td></tr>
-19</span></td><td><a href='/guix/commit/gnu/packages/sync.scm?id=162a13740041f907f0ce5c2aa05b52b162b4e81a'>gnu: Consistently Write ‘file system(s)’.</a><span class='msg-avail'>...</span></td><td>Tobias Geerinckx-Rice</td></tr>
<tr><td><span title='2018-01-10 15:59:47 +0100'>2018-01-10</span></td><td><a href='/guix/commit/gnu/packages/sync.scm?id=9358ae8adf4fc3dbbe4164d0bc16d5ce11b41393'>gnu: lsyncd: Update to 2.2.2.</a><span class='msg-avail'>...</span></td><td>Tobias Geerinckx-Rice</td></tr>
<tr><td><span title='2017-11-18 21:50:11 +0200'>2017-11-18</span></td><td><a href='/guix/commit/gnu/packages/sync.scm?id=5af27a82c3f95eaa560e2771b629a93693c7f442'>gnu: owncloud-client: Update to 2.3.4.</a><span class='msg-avail'>...</span></td><td>Efraim Flashner</td></tr>
<tr><td><span title='2017-11-18 21:50:08 +0200'>2017-11-18</span></td><td><a href='/guix/commit/gnu/packages/sync.scm?id=6563d58c7e94bee8b9beeefa929ab2e982d86fa0'>gnu: owncloud-client: Don't check for updates.</a><span class='msg-avail'>...</span></td><td>Efraim Flashner</td></tr>
<tr><td><span title='2017-09-17 10:14:14 +0300'>2017-09-17</span></td><td><a href='/guix/commit/gnu/packages/sync.scm?id=5fb17ed92e910030b86cc6f1eec50a2c3b725b08'>gnu: owncloud-client: Update to 2.3.3.</a><span class='msg-avail'>...</span></td><td>Efraim Flashner</td></tr>
<tr><td><span title='2017-07-20 00:35:02 +0200'>2017-07-20</span></td><td><a href='/guix/commit/gnu/packages/sync.scm?id=dc05f6523eb424bfeb4ddd2bf157232b4a72ca77'>gnu: qsyncthingtray: Update to 0.5.8.</a><span class='msg-avail'>...</span></td><td>Tobias Geerinckx-Rice</td></tr>
<tr><td><span title='2017-04-26 22:04:01 +0300'>2017-04-26</span></td><td><a href='/guix/commit/gnu/packages/sync.scm?id=be58ffa4aa518686294726bdd83733eca4e6c03c'>gnu: owncloud-client: Update to 2.3.1.</a><span class='msg-avail'>...</span></td><td>Efraim Flashner</td></tr>