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='2018-03-14 17:37:20 +0100'>2018-03-14</span></td><td><a href='/guix/commit/gnu/packages/kodi.scm?id=8c72ed923d77ee55989965bb02628043799b9548'>Merge branch 'master' into core-updates</a></td><td>Ricardo Wurmus</td></tr>
<tr><td><span title='2018-03-11 22:04:40 +0100'>2018-03-11</span></td><td><a href='/guix/commit/gnu/packages/kodi.scm?id=189be331acfda1c242a9c85fca8d2a0356742f48'>build-system/gnu: Add 'bootstrap' phase.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2018-02-25 18:35:25 +0100'>2018-02-25</span></td><td><a href='/guix/commit/gnu/packages/kodi.scm?id=9d2749f62f1479d2d88cb8270fb20ba2bb146b33'>gnu: kodi: Update to 18.0_alpha-8.ec16dbc.</a><span class='msg-avail'>...</span></td><td>Marius Bakke</td></tr>
<tr><td><span title='2017-11-20 00:17:39 +0100'>2017-11-20</span></td><td><a href='/guix/commit/gnu/packages/kodi.scm?id=08cc0ef096ea37d0936b3e2ba2df693aaf52729a'>gnu: kodi: Update snapshot.</a><span class='msg-avail'>...</span></td><td>Marius Bakke</td></tr>
<tr><td><span title='2017-11-03 06:46:03 +0300'>2017-11-03</span></td><td><a href='/guix/commit/gnu/packages/kodi.scm?id=72df48dbad95c3bc70a2962f496420ce3363d0de'>gnu: Add kodi-cli.</a><span class='msg-avail'>...</span></td><td>Oleg Pykhalov</td></tr>