Add support for feedback option LOSS

[Didier Barvaux]

The ROHC library 1.6.x does not support the feedback option LOSS. Option LOSS is defined in RFC 3095, section 5.7.6.9 (https://tools.ietf.org/html/rfc3095#section-5.7.6.9) :

   The LOSS option allows the decompressor to report the largest
   observed number of packets lost in sequence. This information MAY be
   used by the compressor to adjust the size of the reference window
   used in U- and O-mode.

   +---+---+---+---+---+---+---+---+
   | Opt Type = 7 | Opt Len = 1 |
   +---+---+---+---+---+---+---+---+
   | longest loss event (packets) |
   +---+---+---+---+---+---+---+---+

   The decompressor MAY choose to ignore the oldest loss events. Thus,
   the value reported may decrease. Since setting the reference window
   too small can reduce robustness, a FEEDBACK packet carrying a LOSS
   option SHOULD also carry a CRC option. The compressor MAY choose to
   ignore decreasing loss values.