shared/ethtool-util: hush gcc warnings about array bounds

shared/ethtool-util: hush gcc warnings about array bounds

[127/1355] Compiling C object 'src/shared/5afaae1@@systemd-shared-245@sta/ethtool-util.c.o'
../src/shared/ethtool-util.c: In function ‘ethtool_get_permanent_macaddr’:
../src/shared/ethtool-util.c:260:60: warning: array subscript 5 is outside the bounds of an interior zero-length array ‘__u8[0]’ {aka ‘unsigned char[]’} [-Wzero-length-bounds]
  260 |                 ret->ether_addr_octet[i] = epaddr.addr.data[i];
      |                                            ~~~~~~~~~~~~~~~~^~~
In file included from ../src/shared/ethtool-util.c:5:
../src/shared/linux/ethtool.h:704:7: note: while referencing ‘data’
  704 |  __u8 data[0];
      |       ^~~~
../src/shared/ethtool-util.c: In function ‘ethtool_set_features’:
../src/shared/ethtool-util.c:488:31: warning: array subscript 0 is outside the bounds of an interior zero-length array ‘__u32[0]’ {aka ‘unsigned int[]’} [-Wzero-length-bounds]
  488 |         len = buffer.info.data[0];
      |               ~~~~~~~~~~~~~~~~^~~
In file included from ../src/shared/ethtool-util.c:5:
../src/shared/linux/ethtool.h:631:8: note: while referencing ‘data’
  631 |  __u32 data[0];
      |        ^~~~

The kernel should not define the length of the array, but it does. We can't fix
that, so let's use a cast to avoid the warning.

For https://github.com/systemd/systemd/issues/6119#issuecomment-626073743.

v2:
- use #pragma instead of a cast. It seems the cast only works in some cases, and
  gcc is "smart" enough to see beyond the cast. Unfortunately clang does not support
  this warning, so we need to do a config check whether to try to suppress.

(cherry picked from commit 94c0c5b7eaa1529ff694318d3755181ba96ecefb)