HO preparation: fix PLMN type in nr_neighbour_cell_t

During the review of the N2 HO MR it has been found that the wrong type
plmn_identity_t (S1AP) was used in RRC. Now updated to the platform type plmn_id_t.

openair2/RRC/NR/nr_rrc_defs.h

@@ -315,7 +315,7 @@ typedef struct {
   int absoluteFrequencySSB;
   int subcarrierSpacing;
   int band;
-  plmn_identity_t plmn;
+  plmn_id_t plmn;
   uint32_t tac;
   bool isIntraFrequencyNeighbour;
 } nr_neighbour_cell_t;