gnu: valgrind: Fix test failure with glibc 2.26.

* gnu/packages/patches/valgrind-glibc-compat.patch: New file.
* gnu/local.mk (dist_patch_DATA): Add it.
* gnu/packages/valgrind.scm (valgrind)[source](patches): Use it.

gnu/local.mk

@@ -1090,6 +1090,7 @@ dist_patch_DATA =						\
   %D%/packages/patches/util-linux-tests.patch			\
   %D%/packages/patches/upower-builddir.patch			\
   %D%/packages/patches/valgrind-enable-arm.patch		\
+  %D%/packages/patches/valgrind-glibc-compat.patch		\
   %D%/packages/patches/vinagre-revert-1.patch                   \
   %D%/packages/patches/vinagre-revert-2.patch                   \
   %D%/packages/patches/virglrenderer-CVE-2017-6386.patch 	\

gnu/packages/patches/valgrind-glibc-compat.patch

@@ -0,0 +1,18 @@
+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;

gnu/packages/valgrind.scm

@@ -39,7 +39,8 @@
              (sha256
               (base32
                "18bnrw9b1d55wi1wnl68n25achsp9w48n51n1xw4fwjjnaal7jk7"))
-             (patches (search-patches "valgrind-enable-arm.patch"))))
+             (patches (search-patches "valgrind-enable-arm.patch"
+                                      "valgrind-glibc-compat.patch"))))
     (build-system gnu-build-system)
     (outputs '("doc"                              ;16 MB
                "out"))