net: ppp: access ppp->nextseq only if CONFIG_PPP_MULTILINK is defined

Commit d762d038 resets the counter holding the
next sequence number for multilink PPP fragments to zero whenever the
SC_MULTILINK flag is set. However, this counter only exists if
CONFIG_PPP_MULTILINK is defined. Consequently, the new code has to be enclosed
within #ifdef CONFIG_PPP_MULTILINK ... #endif.

Signed-off-by: Christoph Schulz <develop@kristov.de>
Signed-off-by: David S. Miller <davem@davemloft.net>