net/net_pkt: Fix is_ppp dependency issue

The functions net_pkt_is_ppp() and net_pkt_set_ppp() are used by
both the l2 ppp subsystem (NET_L2_PPP) and the ppp uart based driver
(NET_PPP), but the function is only included when NET_PPP is enabled.

the l2 ppp subsystem does not depend on the uart based PPP driver, the
ppp uart driver does however depend on the l2 ppp subsystem. Including
the two functions should therefore be dependent on if the l2 ppp
subsystem is enabled using the symbol CONFIG_NET_L2_PPP instead of the
symbol CONFIG_NET_PPP

Signed-off-by: Bjarki Arge Andreasen <baa@trackunit.com>