rlc_am: fix poll bit during header reconstruction

this makes sure that the reconstructed RLC AM header
has the poll bit set when any of its segments had it set.

this triggered a bug in RLC AM because the Status PDU
isn't transmitted for a RLC PDU that was segmented and
only the first segment had the P flag set.

lib/src/upper/rlc_am_lte.cc

@@ -1842,6 +1842,9 @@ bool rlc_am_lte::rlc_am_lte_rx::add_segment_and_check(rlc_amd_rx_pdu_segments_t*
       carryover = 0;
     }
     count = 0;
+
+    // set Poll bit if any of the segments had it set
+    header.p |= it->header.p;
   }
 
   log->debug("Finished header reconstruction of %zd segments\n", pdu->segments.size());