igc: fix build errors for PTP

When IGC=y and PTP_1588_CLOCK=m, the ptp_*() interface family is
not available to the igc driver. Make this driver depend on
PTP_1588_CLOCK_OPTIONAL so that it will build without errors.

Various igc commits have used ptp_*() functions without checking
that PTP_1588_CLOCK is enabled. Fix all of these here.

Fixes these build errors:

ld: drivers/net/ethernet/intel/igc/igc_main.o: in function `igc_msix_other':
igc_main.c:(.text+0x6494): undefined reference to `ptp_clock_event'
ld: igc_main.c:(.text+0x64ef): undefined reference to `ptp_clock_event'
ld: igc_main.c:(.text+0x6559): undefined reference to `ptp_clock_event'
ld: drivers/net/ethernet/intel/igc/igc_ethtool.o: in function `igc_ethtool_get_ts_info':
igc_ethtool.c:(.text+0xc7a): undefined reference to `ptp_clock_index'
ld: drivers/net/ethernet/intel/igc/igc_ptp.o: in function `igc_ptp_feature_enable_i225':
igc_ptp.c:(.text+0x330): undefined reference to `ptp_find_pin'
ld: igc_ptp.c:(.text+0x36f): undefined reference to `ptp_find_pin'
ld: drivers/net/ethernet/intel/igc/igc_ptp.o: in function `igc_ptp_init':
igc_ptp.c:(.text+0x11cd): undefined reference to `ptp_clock_register'
ld: drivers/net/ethernet/intel/igc/igc_ptp.o: in function `igc_ptp_stop':
igc_ptp.c:(.text+0x12dd): undefined reference to `ptp_clock_unregister'
ld: drivers/platform/x86/dell/dell-wmi-privacy.o: in function `dell_privacy_wmi_probe':

Fixes: 64433e5b ("igc: Enable internal i225 PPS")
Fixes: 60dbede0 ("igc: Add support for ethtool GET_TS_INFO command")
Fixes: 87938851 ("igc: enable auxiliary PHC functions for the i225")
Fixes: 5f295805

 ("igc: Add basic skeleton for PTP")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Ederson de Souza <ederson.desouza@intel.com>
Cc: Tony Nguyen <anthony.l.nguyen@intel.com>
Cc: Vinicius Costa Gomes <vinicius.gomes@intel.com>
Cc: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: Jesse Brandeburg <jesse.brandeburg@intel.com>
Cc: intel-wired-lan@lists.osuosl.org
Acked-by: Vinicius Costa Gomes <vinicius.gomes@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>