selftests/vm: remove __USE_GNU in hugetlb-madvise.c

selftests/vm: remove __USE_GNU in hugetlb-madvise.c

__USE_GNU should be an internal macro only used inside glibc.  Either
memfd_create() or fallocate() requires _GNU_SOURCE per man page, where
__USE_GNU will further be defined by glibc headers include/features.h:

  #ifdef _GNU_SOURCE
  # define __USE_GNU	1
  #endif

This fixes:

   >> hugetlb-madvise.c:20: warning: "__USE_GNU" redefined
      20 | #define __USE_GNU
         |
   In file included from /usr/include/x86_64-linux-gnu/bits/libc-header-start.h:33,
                    from /usr/include/stdlib.h:26,
                    from hugetlb-madvise.c:16:
   /usr/include/features.h:407: note: this is the location of the previous definition
     407 | # define __USE_GNU      1
         |

Link: https://lkml.kernel.org/r/Y8V9z+z6Tk7NetI3@x1n
Signed-off-by: Peter Xu <peterx@redhat.com>
Reported-by: kernel test robot <lkp@intel.com>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>