lib/support: clean up definition of flags_array

lib/support: clean up definition of flags_array

Add braces to address the following compiler warning with gcc -Wall:

print_fs_flags.c:24:42: warning: missing braces around initializer [-Wmissing-braces]
   24 | static struct flags_name flags_array[] = {
      |                                          ^

Also add 'const', and add an explicit NULL in the last entry.

Reviewed-by: Lukas Czerner <lczerner@redhat.com>
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>