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='2017-01-29 23:46:33 +0100'>2017-01-29</span></td><td><a href='/guix/commit/gnu/packages/ldc.scm?id=4fdcc5026fb6a1fe007363eb32af891f35704e28'>gnu: Add dub.</a><span class='msg-avail'>...</span></td><td>Danny Milosavljevic</td></tr>
<tr><td><span title='2017-01-28 01:25:55 +0100'>2017-01-28</span></td><td><a href='/guix/commit/gnu/packages/ldc.scm?id=2429dde57d8461cc8e51dbe6c9d47d795fe6fc9a'>gnu: Add ldc@1.1.0-beta6.</a><span class='msg-avail'>...</span></td><td>Muriithi Frederick Muriuki</td></tr>
<tr><td><span title='2016-11-25 23:18:29 -0500'>2016-11-25</span></td><td><a href='/guix/commit/gnu/packages/ldc.scm?id=0959478c726a3117da815f47c0ce92e4588a8ac4'>gnu: Add missing module import to (gnu packages ldc).</a><span class='msg-avail'>...</span></td><td>Leo Famulari</td></tr>
<tr><td><span title='2016-11-25 23:54:51 +0100'>2016-11-25</span></td><td><a href='/guix/commit/gnu/packages/ldc.scm?id=e44b511298590ecc87c2c85d1cbc043a638dd1e0'>gnu: ldc: Update to 0.17.2.</a><span class='msg-avail'>...</span></td><td>Danny Milosavljevic</td></tr>
<tr><td><span title='2016-09-13 15:24:55 -0400'>2016-09-13</span></td><td><a href='/guix/commit/gnu/packages/ldc.scm?id=812b3c1f8a3717fd9fc540663912c5e9a144db5b'>gnu: ldc: Fix build failure.</a><span class='msg-avail'>...</span></td><td>Danny Milosavljevic</td></tr>
<tr><td><span title='2016-04-14 10:38:21 +0300'>2016-04-14</span></td><td><a href='/guix/commit/gnu/packages/ldc.scm?id=fc1adab1487e5ea3c5f6341263df14414ba4e784'>gnu: packages: Use 'search-patches' everywhere.</a><span class='msg-avail'>...</span></td><td>Alex Kost</td></tr>
<tr><td><span title='2016-02-23 01:27:19 -0500'>2016-02-23</span></td><td><a href='/guix/commit/gnu/packages/ldc.scm?id=c1fc7a67e45bb4df944e3c2dd0cdb21a0ecde478'>gnu: Add rdmd.</a><span class='msg-avail'>...</span></td><td>Roel Janssen</td></tr>
<tr><td><span title='2016-01-06 14:21:53 +0100'>2016-01-06</span></td><td><a href='/guix/commit/gnu/packages/ldc.scm?id=341ae582134e1d0393db864584c42228341ac3ec'>gnu: ldc: Fix license.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2016-01-05 16:24:57 +0100'>2016-01-05</span></td><td><a href='/guix/commit/gnu/packages/ldc.scm?id=a7ec569c98af8a1c310a32e19f0c01cdd7334af6'>gnu: Add LDC.</a><span class='msg-avail'>...</span></td><td>Roel Janssen</td></tr>