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/src/gb/gprs_bssgp.c b/src/gb/gprs_bssgp.c
index c967f73..7abef80 100644
--- a/src/gb/gprs_bssgp.c
+++ b/src/gb/gprs_bssgp.c
@@ -1428,7 +1428,7 @@
 /*!
  * \brief Flush the queues of all BSSGP contexts.
  */
-void bssgp_flush_all_queues()
+void bssgp_flush_all_queues(void)
 {
 	struct bssgp_bvc_ctx *bctx;