scripts/Makefile.clean: clean also $(extra-m) and $(extra-)

This commit is a backport from Linux Kernel,
commit 9d5db8949f1ecf4019785b04d8986835d3c0e99e,
written by me.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
1 file changed