f_u32: fix compiler gcc-10 compiler warning

f_u32: fix compiler gcc-10 compiler warning

With gcc-10 it complains about array subscript error.

f_u32.c: In function ‘u32_parse_opt’:
f_u32.c:1113:24: warning: array subscript 0 is outside the bounds of an interior zero-length array ‘struct tc_u32_key[0]’ [-Wzero-length-bounds]
 1113 |    hash = sel2.sel.keys[0].val & sel2.sel.keys[0].mask;
      |           ~~~~~~~~~~~~~^~~
In file included from tc_util.h:11,
                 from f_u32.c:26:
../include/uapi/linux/pkt_cls.h:253:20: note: while referencing ‘keys’
  253 |  struct tc_u32_key keys[0];
      |

This is because the keys are actually allocated in the second element
of the parent structure.

Simplest way to address the warning is to assign directly to the keys
in the containing structure.

This has always been in iproute2 (pre-git) so no Fixes.

Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>