Support building with -Werror=strict-prototypes / -Werror=old-style-definition

Unfortunately "-std=c99" is not sufficient to make gcc ignore code that
uses constructs of earlier C standards, which were abandoned in C99.

See https://lwn.net/ml/fedora-devel/Y1kvF35WozzGBpc8@redhat.com/ for
some related discussion.

Change-Id: I84fd99442d0cc400fa562fa33623c142649230e2
diff --git a/include/osmocom/gprs/gprs_bssgp.h b/include/osmocom/gprs/gprs_bssgp.h
index 09f640d..6c04332 100644
--- a/include/osmocom/gprs/gprs_bssgp.h
+++ b/include/osmocom/gprs/gprs_bssgp.h
@@ -235,7 +235,7 @@
 int bssgp_fc_ms_init(struct bssgp_flow_control *fc_ms, uint16_t bvci,
 		     uint16_t nsei, uint32_t max_queue_depth);
 
-void bssgp_flush_all_queues();
+void bssgp_flush_all_queues(void);
 void bssgp_fc_flush_queue(struct bssgp_flow_control *fc);
 
 /* gprs_bssgp_vty.c */