slirp/arp_table.c: Avoid shifting into sign bit of signed integers

"0xf << 28" shifts right into the sign bit, since 0xf is a signed
integer. Use the 'U' suffix to force an unsigned shift to avoid
this undefined behaviour and a clang sanitizer warning.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Eric Blake <eblake@redhat.com>
Acked-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
