unix: add PTP_PF_* constants on Linux

These represent valid values for PtpPinDesc.Func.
They are defined as enum, thus not emitted from mkerrors.sh.

Change-Id: Ibac22d56214fa0f64a4095dfc000692a5e5e994a
GitHub-Last-Rev: e7990b0
GitHub-Pull-Request: #229
Reviewed-on: https://go-review.googlesource.com/c/sys/+/621735
Reviewed-by: Michael Pratt <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>
Auto-Submit: Ian Lance Taylor <[email protected]>

Author: yarikk

unix/linux/types.go

@@ -4148,6 +4148,13 @@ type (
 	PtpSysOffsetPrecise C.struct_ptp_sys_offset_precise
 )
 
+const (
+	PTP_PF_NONE    = C.PTP_PF_NONE
+	PTP_PF_EXTTS   = C.PTP_PF_EXTTS
+	PTP_PF_PEROUT  = C.PTP_PF_PEROUT
+	PTP_PF_PHYSYNC = C.PTP_PF_PHYSYNC
+)
+
 type (
 	HIDRawReportDescriptor C.struct_hidraw_report_descriptor
 	HIDRawDevInfo          C.struct_hidraw_devinfo