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='2021-12-13 17:48:25 +0100'>2021-12-13</span></td><td><a href='/guix/commit/gnu/packages/kawa.scm?id=8394619baceb118df92e355377fd543bb1aa501a'>gnu: Simplify package inputs.</a><span class='msg-avail'>...<span class='msg-tooltip'>This commit was obtained by running:

  ./pre-inst-env guix style

without any additional argument.
</span></span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2020-04-14 07:07:04 +0200'>2020-04-14</span></td><td><a href='/guix/commit/gnu/packages/kawa.scm?id=c206b8398541d052d506c57f627c5c3c7d26686b'>gnu: kawa: Update to 3.1.1.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/kawa.scm (kawa): Update to 3.1.1.
</span></span></td><td>Tobias Geerinckx-Rice</td></tr>
<tr><td><span title='2020-04-14 07:07:04 +0200'>2020-04-14</span></td><td><a href='/guix/commit/gnu/packages/kawa.scm?id=ea607b355169158fefa796cabb3add66aac7301d'>gnu: kawa: Wrap to find java.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/kawa.scm (kawa)[arguments]: Add ‘wrap-kawa’ phase to set JAVA_HOME.
</span></span></td><td>Tobias Geerinckx-Rice</td></tr>
<tr><td><span title='2019-06-13 13:00:37 +0200'>2019-06-13</span></td><td><a href='/guix/commit/gnu/packages/kawa.scm?id=47f2168b6fabb105565526b2a1243eeeb13008fe'>gnu: Add Kawa.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/kawa.scm: New file.
* gnu/local.mk (GNU_SYSTEM_MODULES): Add it.
</span></span></td><td>Carlo Zancanaro</td></tr>