net: dsa: ocelot: add PTP dependency for NET_DSA_MSCC_OCELOT_EXT

A new user of MSCC_OCELOT_SWITCH_LIB was added, bringing back an old
link failure that was fixed with e5f31552674e ("ethernet: fix
PTP_1588_CLOCK dependencies"):

x86_64-linux-ld: drivers/net/ethernet/mscc/ocelot_ptp.o: in function `ocelot_ptp_enable':
ocelot_ptp.c:(.text+0x8ee): undefined reference to `ptp_find_pin'
x86_64-linux-ld: drivers/net/ethernet/mscc/ocelot_ptp.o: in function `ocelot_get_ts_info':
ocelot_ptp.c:(.text+0xd5d): undefined reference to `ptp_clock_index'
x86_64-linux-ld: drivers/net/ethernet/mscc/ocelot_ptp.o: in function `ocelot_init_timestamp':
ocelot_ptp.c:(.text+0x15ca): undefined reference to `ptp_clock_register'
x86_64-linux-ld: drivers/net/ethernet/mscc/ocelot_ptp.o: in function `ocelot_deinit_timestamp':
ocelot_ptp.c:(.text+0x16b7): undefined reference to `ptp_clock_unregister'

Add the same PTP dependency here, as well as in the MSCC_OCELOT_SWITCH_LIB
symbol itself to make it more obvious what is going on when the next
driver selects it.

Fixes: 3d7316ac81ac ("net: dsa: ocelot: add external ocelot switch control")
Signed-off-by: Arnd Bergmann <[e-mail]>
Acked-by: Colin Foster <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

drivers/net/dsa/ocelot/Kconfig

···
 	tristate "Ocelot External Ethernet switch support"
 	depends on NET_DSA && SPI
 	depends on NET_VENDOR_MICROSEMI
+	depends on PTP_1588_CLOCK_OPTIONAL
 	select MDIO_MSCC_MIIM
 	select MFD_OCELOT_CORE
 	select MSCC_OCELOT_SWITCH_LIB

drivers/net/ethernet/mscc/Kconfig

···
 
 # Users should depend on NET_SWITCHDEV, HAS_IOMEM, BRIDGE
 config MSCC_OCELOT_SWITCH_LIB
+	depends on PTP_1588_CLOCK_OPTIONAL
 	select NET_DEVLINK
 	select REGMAP_MMIO
 	select PACKING