Define timestruc_t for Lustre compatibility

Lustre 2.8 (and possibly other versions) are still using timestruc_t,
which was removed in spl-0.7.10 in favor of inode_timespec_t.  Add
in a backwards compatibility #define for timestruc_t so that Lustre
builds.

Signed-off-by: Tony Hutter <hutter2@llnl.gov>

include/sys/time.h

@@ -66,6 +66,9 @@ typedef struct timespec64	inode_timespec_t;
 typedef struct timespec		inode_timespec_t;
 #endif
 
+/* Include for Lustre compatibility */
+#define timestruc_t	inode_timespec_t
+
 static inline void
 gethrestime(inode_timespec_t *ts)
  {