net: net_tstamp: Add unspec field to hwtstamp_source enumeration

Prepare for future support of saving hwtstamp source in PTP xarray by
introducing HWTSTAMP_SOURCE_UNSPEC to hwtstamp_source enum, setting it
to 0 to match old behavior of no source defined.
Signed-off-by: Kory Maincent <kory.maincent@bootlin.com>
Link: https://patch.msgid.link/20240709-feature_ptp_netnext-v17-5-b5317f50df2a@bootlin.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

include/linux/net_tstamp.h

@@ -14,6 +14,7 @@
 					 SOF_TIMESTAMPING_RAW_HARDWARE)
 
 enum hwtstamp_source {
+	HWTSTAMP_SOURCE_UNSPEC,
 	HWTSTAMP_SOURCE_NETDEV,
 	HWTSTAMP_SOURCE_PHYLIB,
 };