selftests/memfd: Fix implicit declaration warnings

The memfd tests emit several warnings:

  fuse_test.c:261:7: warning: implicit declaration of function 'open'
  fuse_test.c:67:6: warning: implicit declaration of function 'fcntl'
  memfd_test.c:397:6: warning: implicit declaration of function 'fallocate'
  memfd_test.c:64:7: warning: implicit declaration of function 'open'
  memfd_test.c:90:6: warning: implicit declaration of function 'fcntl'

These are all caused by the test not including fcntl.h.

Instead of including linux/fcntl.h, include fcntl.h, which should
eventually cause the former to be included as well.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>