selftests/mount_setattr: fix redefine struct mount_attr build error

Fix the following build error due to redefining struct mount_attr by
removing duplicate define from mount_setattr_test.c

gcc -g -isystem .../tools/testing/selftests/../../../usr/include -Wall -O2 -pthread     mount_setattr_test.c  -o .../tools/testing/selftests/mount_setattr/mount_setattr_test
mount_setattr_test.c:107:8: error: redefinition of ‘struct mount_attr’
  107 | struct mount_attr {
      |        ^~~~~~~~~~
In file included from /usr/include/x86_64-linux-gnu/sys/mount.h:32,
                 from mount_setattr_test.c:10:
.../usr/include/linux/mount.h:129:8: note: originally defined here
  129 | struct mount_attr {
      |        ^~~~~~~~~~
make: *** [../lib.mk:145: .../tools/testing/selftests/mount_setattr/mount_setattr_test] Error 1

Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>