module/Makefile.in: don't run xargs if empty

If stdin if empty - don't run xargs command,
otherwise we can get `cp: missing file operand`
error.

Reviewed-by: Ryan Moeller <ryan@ixsystems.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: George Melikov <mail@gmelikov.ru>
Closes #9418

module/Makefile.in

@@ -115,5 +115,5 @@ distdir:
 	list='$(obj-m)'; for objdir in $$list; do \
 		(cd @top_srcdir@/module && find $$objdir \
 		-name '*.c' -o -name '*.h' -o -name '*.S' | \
-		xargs cp --parents -t @abs_top_builddir@/module/$$distdir); \
+		xargs -r cp --parents -t @abs_top_builddir@/module/$$distdir); \
 	done