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 <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

include/linux/net_tstamp.h

···
 					 SOF_TIMESTAMPING_RAW_HARDWARE)
 
 enum hwtstamp_source {
+	HWTSTAMP_SOURCE_UNSPEC,
 	HWTSTAMP_SOURCE_NETDEV,
 	HWTSTAMP_SOURCE_PHYLIB,
 };