statistics: Introduce 'struct counter' instead of using unsigned long

This has the advantage that counters can be added all over the code
very easily, while having only one routine that stores all of the
current counter values to the database.  The counters are synced
every 60 seconds, providing relatively fine grained statistics
about the network usage as time passes by.
diff --git a/openbsc/src/bs11_config.c b/openbsc/src/bs11_config.c
index 3e8bf88..2a80a49 100644
--- a/openbsc/src/bs11_config.c
+++ b/openbsc/src/bs11_config.c
@@ -71,6 +71,11 @@
 
 static const u_int8_t too_fast[] = { 0x12, 0x80, 0x00, 0x00, 0x02, 0x02 };
 
+/* dummy function to keep gsm_data.c happy */
+struct counter *counter_alloc(const char *name)
+{
+	return NULL;
+}
 
 int handle_serial_msg(struct msgb *rx_msg);