fix compile error when USE_NSEC is defined

'struct cache' does not have a 'usec' member, but a 'unsigned int
nsec' member.  Simmilar 'struct stat' does not have a 'st_mtim.usec'
member, and we should instead use 'st_mtim.tv_nsec'.

Signed-off-by: Kjetil Barvik <barvik@broadpark.no>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-fetch-pack.c

@@ -802,14 +802,14 @@ struct ref *fetch_pack(struct fetch_pack_args *my_args,
 
 		mtime.sec = st.st_mtime;
 #ifdef USE_NSEC
-		mtime.usec = st.st_mtim.usec;
+		mtime.nsec = st.st_mtim.tv_nsec;
 #endif
 		if (stat(shallow, &st)) {
 			if (mtime.sec)
 				die("shallow file was removed during fetch");
 		} else if (st.st_mtime != mtime.sec
 #ifdef USE_NSEC
-				|| st.st_mtim.usec != mtime.usec
+				|| st.st_mtim.tv_nsec != mtime.nsec
 #endif
 			  )
 			die("shallow file was changed during fetch");