gprs_ms: Small clean ups in IMSI storage related code

Change-Id: I987af0d33b79302c037d062c9d1c828a0e027147
diff --git a/src/gprs_ms.h b/src/gprs_ms.h
index f7b5c36..1e5a155 100644
--- a/src/gprs_ms.h
+++ b/src/gprs_ms.h
@@ -32,6 +32,8 @@
 	#include <osmocom/core/timer.h>
 	#include <osmocom/core/linuxlist.h>
 
+	#include <osmocom/gsm/protocol/gsm_23_003.h>
+
 	#include "coding_scheme.h"
 }
 
@@ -159,7 +161,7 @@
 	uint32_t m_new_dl_tlli;
 
 	/* store IMSI for look-up and PCH retransmission */
-	char m_imsi[16];
+	char m_imsi[OSMO_IMSI_BUF_SIZE];
 	uint8_t m_ta;
 	uint8_t m_ms_class;
 	uint8_t m_egprs_ms_class;