gsm_08_16.h: Add missing header

utils.h is needed for struct value_string
This probably never caused a problem because every file including
gsm_08_16.h also included utils.h, but we should still include the file
here.

Change-Id: Iae09b4e8e42be6c371fb34279b7981db2af8cf4c
diff --git a/include/osmocom/gprs/protocol/gsm_08_16.h b/include/osmocom/gprs/protocol/gsm_08_16.h
index e035ebf..f98f68d 100644
--- a/include/osmocom/gprs/protocol/gsm_08_16.h
+++ b/include/osmocom/gprs/protocol/gsm_08_16.h
@@ -7,6 +7,7 @@
 
 #include <stdint.h>
 #include <arpa/inet.h>
+#include <osmocom/core/utils.h>
 
 /*! \addtogroup libgb
  *  @{