Add are to ldms_set_hdr for compatible updates (#103)

Reserve an area in the set hdr to accomodate changes
that may affect this structure but still support backward
compatability.

ldms/src/core/ldms_core.h

@@ -202,6 +202,7 @@ struct ldms_set_hdr {
 	uint32_t gid;           /* GID */
 	uint32_t perm;          /* permission */
 	uint32_t array_card;    /* number of sets in the set array */
+	uint32_t reserved[8];	/* area reserved for compatible core updates */
 	uint32_t dict[OVIS_FLEX];/* The attr/metric dictionary */
 };