tools/testing/selftests/ptrace/peeksiginfo.c: add PAGE_SIZE definition

tools/testing/selftests/ptrace/peeksiginfo.c: add PAGE_SIZE definition

On IBM powerpc where multiple page size value are supported, current
ppc64 and ppc64el distro don't define the PAGE_SIZE variable in
/usr/include as this is a dynamic value retrieved by the getpagesize()
or sysconf() defined in unistd.h.  The PAGE_SIZE variable sounds defined
when only one value is supported by the kernel.

As such, when the PAGE_SIZE definition doesn't exist system should
retrieve the dynamic value.

Signed-off-by: Thierry Fauck <thierry@linux.vnet.ibm.com>
Cc: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>