uapi: move IPPROTO_L2TP to in.h

IPPROTO_L2TP is currently defined in l2tp.h, but most of
ip protocols are defined in in.h file. Move it there in order
to keep code clean.

Acked-by: Guillaume Nault <gnault@redhat.com>
Signed-off-by: Wojciech Drewek <wojciech.drewek@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>