sms: Mark the state dispatch table as static const

GCC 4.7.2 was already smart enough to see that the table is const
so there is no change in the generated assembly code. For some reason
the dispatch is still going through one relocation.
diff --git a/src/gsm/gsm0411_smr.c b/src/gsm/gsm0411_smr.c
index 8003bc8..c6c85e1 100644
--- a/src/gsm/gsm0411_smr.c
+++ b/src/gsm/gsm0411_smr.c
@@ -369,7 +369,7 @@
 }
 
 /* statefull handling for SM-RL SAP messages */
-static struct smrdownstate {
+static const struct smrdownstate {
 	uint32_t	states;
 	int		type;
 	const char 	*name;
@@ -421,7 +421,7 @@
 }
 
 /* statefull handling for MMSMS SAP messages */
-static struct smrdatastate {
+static const struct smrdatastate {
 	uint32_t	states;
 	int		type;
 	const char 	*name;