zdtm: fix comilation for non-constant PAGE_SIZE

On some systems PAGE_SIZE is declared as sysconf(_SC_PAGESIZE) in <sys/user.h>
this is non-constant expression, so it cannot be used in type declarations.

cc -g -O2 -Wall -Werror -U_FORTIFY_SOURCE -D_FORTIFY_SOURCE=0   -c -o ns.o ns.c
ns.c:124:7: error: variably modified ‘stack’ at file scope

Signed-off-by: Konstantin Khlebnikov <khlebnikov@openvz.org>
Acked-by: Andrew Vagin <avagin@parallels.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>