compel/s390: glibc renamed ucontext to ucontext_t

The upcoming glibc release renamed 'struct ucontext' to
'struct ucontext_t':

https://sourceware.org/git/gitweb.cgi?p=glibc.git;a=commitdiff;h=251287734e89a52da3db682a8241eb6bccc050c9;hp=c86ed71d633c22d6f638576f7660c52a5f783d66

Instead of using 'struct ucontext' this patch changes it
to the typedef ucontext_t which already exists in older and
new versions of glibc.
Signed-off-by: Adrian Reber <areber@redhat.com>
Reviewed-by: Dmitry Safonov <dsafonov@virtuozzo.com>
Reviewed-by: Michael Holzheu <holzheu@linux.vnet.ibm.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>

compel/arch/s390/src/lib/include/uapi/asm/sigframe.h

@@ -37,7 +37,7 @@ typedef struct {
  */
 struct ucontext_extended {
 	unsigned long     uc_flags;
-	struct ucontext  *uc_link;
+	ucontext_t       *uc_link;
 	stack_t           uc_stack;
 	_sigregs          uc_mcontext;
 	sigset_t          uc_sigmask;