add missing header include

diff --git a/src/gsm_data.c b/src/gsm_data.c
index dd57142..d0ce168 100644
--- a/src/gsm_data.c
+++ b/src/gsm_data.c
@@ -20,6 +20,7 @@
 
 
 #include <stdlib.h>
+#include <stdio.h>
 #include <string.h>
 
 #include <openbsc/gsm_data.h>