net: ti: icssg-prueth: Add optional dependency on HSR

Commit 95540ad6747c ("net: ti: icssg-prueth: Add support for HSR frame
forward offload") introduced support for offloading HSR frame forwarding,
which relies on functions such as is_hsr_master() provided by the HSR
module. Although HSR provides stubs for configurations with HSR
disabled, this driver still requires an optional dependency on HSR.
Otherwise, build failures will occur when icssg-prueth is built-in
while HSR is configured as a module.
ld.lld: error: undefined symbol: is_hsr_master
>>> referenced by icssg_prueth.c:710 (drivers/net/ethernet/ti/icssg/icssg_prueth.c:710)
>>> drivers/net/ethernet/ti/icssg/icssg_prueth.o:(icssg_prueth_hsr_del_mcast) in archive vmlinux.a
>>> referenced by icssg_prueth.c:681 (drivers/net/ethernet/ti/icssg/icssg_prueth.c:681)
>>> drivers/net/ethernet/ti/icssg/icssg_prueth.o:(icssg_prueth_hsr_add_mcast) in archive vmlinux.a
>>> referenced by icssg_prueth.c:1812 (drivers/net/ethernet/ti/icssg/icssg_prueth.c:1812)
>>> drivers/net/ethernet/ti/icssg/icssg_prueth.o:(prueth_netdevice_event) in archive vmlinux.a

ld.lld: error: undefined symbol: hsr_get_port_ndev
>>> referenced by icssg_prueth.c:712 (drivers/net/ethernet/ti/icssg/icssg_prueth.c:712)
>>> drivers/net/ethernet/ti/icssg/icssg_prueth.o:(icssg_prueth_hsr_del_mcast) in archive vmlinux.a
>>> referenced by icssg_prueth.c:712 (drivers/net/ethernet/ti/icssg/icssg_prueth.c:712)
>>> drivers/net/etherneteth_hsr_del_mcast) in archive vmlinux.a
>>> referenced by icssg_prueth.c:683 (drivers/net/ethernet/ti/icssg/icssg_prueth.c:683)
>>> drivers/net/ethernet/ti/icssg/icssg_prueth.o:(icssg_prueth_hsr_add_mcast) in archive vmlinux.a
>>> referenced 1 more times

Fixes: 95540ad6747c ("net: ti: icssg-prueth: Add support for HSR frame forward offload")
Signed-off-by: Kevin Hao <haokexin@gmail.com>
Cc: stable@vger.kernel.org
Link: https://patch.msgid.link/20260207-icssg-dep-v3-1-8c47c1937f81@gmail.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>