added 180 deg. inverted carrier phase PLL lock flag in gnss_synchro

docs/protobuf/gnss_synchro.proto

@@ -36,6 +36,7 @@ message GnssSynchro {
    double rx_time = 23;  // Receiving time after the start of the week, in s
    bool flag_valid_pseudorange = 24;  // Pseudorange computation status
    double interp_tow_ms = 25;  // Interpolated time of week, in ms
+   bool flag_PLL_180_deg_phase_locked = 26; // PLL lock at 180º
 }
 
 /* Observables represents a collection of GnssSynchro annotations */