Add missing extern keyword
GCC assumes extern, but other toolchains complain about multiple variable definition at files including this header.
@@ -94,9 +94,9 @@ extern struct tsch_asn_divisor_t tsch_hopping_sequence_length;
/* TSCH timeslot timing (in rtimer ticks) */
extern rtimer_clock_t tsch_timing[tsch_ts_elements_count];
/* Statistics on the current session */
-unsigned long tx_count;
-unsigned long rx_count;
-unsigned long sync_count;
+extern unsigned long tx_count;
+extern unsigned long rx_count;
+extern unsigned long sync_count;
/* TSCH processes */
PROCESS_NAME(tsch_process);
