[sms] Fix compile warning by including gsm_data.h

In file included from sms_test.c:27:
../../include/openbsc/gsm_utils.h:33: warning: `enum gsm_band' declared inside parameter list
../../include/openbsc/gsm_utils.h:33: warning: its scope is only this definition or declaration, which is probably not what you want
../../include/openbsc/gsm_utils.h:34: warning: `enum gsm_band' declared inside parameter list

openbsc/tests/sms/sms_test.c

@@ -24,6 +24,7 @@
 #include <sys/types.h>
 #include <openbsc/debug.h>
 #include <openbsc/msgb.h>
+#include <openbsc/gsm_data.h>
 #include <openbsc/gsm_utils.h>
 
 int main(int argc, char** argv)