gsm_data: Address compiler warning of unhandled switch

gcc does not really know the _NR_OF_ELEMENTS_IN_ENUM approach, add
the _NUM_GSM_BTS_TYPE to the handled cases.

gsm_data.c: In function ‘gsm_set_bts_type’:
gsm_data.c:349:2: warning: enumeration value ‘_NUM_GSM_BTS_TYPE’ not handled in switch [-Wswitch]

openbsc/src/libcommon/gsm_data.c

@@ -362,6 +362,7 @@ int gsm_set_bts_type(struct gsm_bts *bts, enum gsm_bts_type type)
 	case GSM_BTS_TYPE_BS11:
 	case GSM_BTS_TYPE_UNKNOWN:
 	case GSM_BTS_TYPE_NOKIA_SITE:
+	case _NUM_GSM_BTS_TYPE:
 		break;
 	}