compel/ppc64: 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>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>