src: include new file osmocom/vty/misc.h for vty_out_rate_ctr_group()

This uses the new include file for vty_out_rate_ctr_group() which
available in libosmocore.
diff --git a/openbsc/src/gprs/sgsn_vty.c b/openbsc/src/gprs/sgsn_vty.c
index 7100b1f..cfde630 100644
--- a/openbsc/src/gprs/sgsn_vty.c
+++ b/openbsc/src/gprs/sgsn_vty.c
@@ -36,6 +36,7 @@
 
 #include <osmocom/vty/command.h>
 #include <osmocom/vty/vty.h>
+#include <osmocom/vty/misc.h>
 
 #include <pdp.h>