fix gcc (<= 5) warning with type-limits by casting vr_r to a signed type

this warning is a bit annoying as it is really safe to do
that kind of comparision, it's just that for vr_r==0, any other
unsigned number is >= 0.

lib/src/upper/rlc_am.cc

@@ -1784,9 +1784,7 @@ bool rlc_am::rlc_am_rx::add_segment_and_check(rlc_amd_rx_pdu_segments_t *pdu, rl
 
 bool rlc_am::rlc_am_rx::inside_rx_window(const int16_t sn)
 {
-  if(RX_MOD_BASE(sn) >= RX_MOD_BASE(vr_r) &&
-     RX_MOD_BASE(sn) <  RX_MOD_BASE(vr_mr))
-  {
+  if (RX_MOD_BASE(sn) >= RX_MOD_BASE(static_cast<int16_t>(vr_r)) && RX_MOD_BASE(sn) < RX_MOD_BASE(vr_mr)) {
     return true;
   } else {
     return false;