add missing #include file to sysinfo.h

(otherwise 'struct value_string' would not be defined)
diff --git a/include/osmocom/gsm/sysinfo.h b/include/osmocom/gsm/sysinfo.h
index a66f3f1..6c81134 100644
--- a/include/osmocom/gsm/sysinfo.h
+++ b/include/osmocom/gsm/sysinfo.h
@@ -1,6 +1,7 @@
 #ifndef _OSMO_GSM_SYSINFO_H
 #define _OSMO_GSM_SYSINFO_H
 
+#include <osmocom/core/utils.h>
 #include <osmocom/gsm/protocol/gsm_04_08.h>
 
 enum osmo_sysinfo_type {