eth: fbnic: add CONFIG_PTP_1588_CLOCK_OPTIONAL dependency

fbnic fails to link as built-in when PTP support is in a loadable
module:

aarch64-linux-ld: drivers/net/ethernet/meta/fbnic/fbnic_ethtool.o: in function `fbnic_get_ts_info':
fbnic_ethtool.c:(.text+0x428): undefined reference to `ptp_clock_index'
aarch64-linux-ld: drivers/net/ethernet/meta/fbnic/fbnic_time.o: in function `fbnic_time_start':
fbnic_time.c:(.text+0x820): undefined reference to `ptp_schedule_worker'
aarch64-linux-ld: drivers/net/ethernet/meta/fbnic/fbnic_time.o: in function `fbnic_ptp_setup':
fbnic_time.c:(.text+0xa68): undefined reference to `ptp_clock_register'

Add the appropriate dependency to enforce this.

Fixes: 6a2b3ede ("eth: fbnic: add RX packets timestamping support")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Vadim Fedorenko <vadim.fedorenko@linux.dev>
Message-ID: <20241016062303.2551686-1-arnd@kernel.org>
Signed-off-by: Andrew Lunn <andrew@lunn.ch>