qemu: busybox.mk: fix distclean typo
qemu: busybox.mk: fix distclean typo

Typo causes mrproper to not distclean busybox.

	$ make mrproper
	...
	make -C busybox disclean
	make[1]: Entering directory '/home/gportay/src/templates-for-int/makefile/qemu/busybox-1.27.2'
	make[1]: *** No rule to make target 'disclean'.  Stop.
	make[1]: Leaving directory '/home/gportay/src/templates-for-int/makefile/qemu/busybox-1.27.2'
	make: [busybox.mk:83: busybox_mrproper] Error 2 (ignored)
	...

gazoo74/templates,gazoo74/templates,gazoo74/templates,gazoo74/templates