selftest: fix build error in tools/testing/selftests/vm/userfaultfd.c

When backporting 0db282ba2c12 ("selftest: use mmap instead of
posix_memalign to allocate memory") to this stable branch, I forgot a {
breaking the build.

Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>