userfaultfd: selftest: fix compiler warning

Fixes following compiler warning

userfaultfd.c: In function ‘usage’:
userfaultfd.c:126:2: warning: format not a string literal and no format
	arguments [-Wformat-security]
  fprintf(stderr, examples);

Signed-off-by: Alakesh Haloi <alakesh.haloi@gmail.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Mike Rapoport <rppt@linux.ibm.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>