pc-bios/s390-ccw/netboot.mak: Ignore Clang's warnings about GNU extensions

When compiling the s390-ccw bios with Clang (v14.0), there is currently
an unuseful warning like this:

  CC      pc-bios/s390-ccw/ipv6.o
 ../../roms/SLOF/lib/libnet/ipv6.c:447:18: warning: variable length array
  folded to constant array as an extension [-Wgnu-folding-constant]
                unsigned short raw[ip6size];
                               ^

SLOF is currently GCC-only and cannot be compiled with Clang yet, so
it is expected that such extensions sneak in there - and as long as
we don't want to compile the code with a compiler that is neither GCC
or Clang, it is also not necessary to avoid such extensions.

Thus these GNU-extension related warnings are completely useless in
the s390-ccw bios, especially in the code that is coming from SLOF,
so we should simply disable the related warnings here now.

Message-Id: <20220704111903.62400-13-thuth@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>