Mark write_record static

The write_record() function is private and should be marked as such.

Reviewed-by: Allan Jude <allanjude@freebsd.org>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Matt Macy <mmacy@FreeBSD.org>
Closes #9665

module/zfs/dmu_diff.c

@@ -50,7 +50,7 @@ typedef struct dmu_diffarg {
 	dmu_diff_record_t da_ddr;
 } dmu_diffarg_t;
 
-int
+static int
 write_record(dmu_diffarg_t *da)
 {
 	zfs_file_t *fp;