kbuild: delete include/generated directory by "make mrproper"
Otherwise, "make mrproper" misses to delete some generated files
such as include/generated/compile.h.
Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>