bssgp: Increment BSSGP flow control tag value

Currently the tag value in FLOW CONTROL BVC messages is always 1.

This commit changes the implementation to increment that value with
each of the FLOW CONTROL BVC messages that is sent to the SGSN.

Sponsored-by: On-Waves ehf
diff --git a/src/gprs_bssgp_pcu.h b/src/gprs_bssgp_pcu.h
index aaaff7f..32be765 100644
--- a/src/gprs_bssgp_pcu.h
+++ b/src/gprs_bssgp_pcu.h
@@ -61,6 +61,7 @@
 	/* Flow control */
 	struct timeval queue_delay_sum;
 	unsigned queue_delay_count;
+	uint8_t fc_tag;
 
 	/** callbacks below */