kbuild: include $(src)/Makefile rather than $(obj)/Makefile

This commit actually has no impact because $(src) and $(obj) point
to the same path, but $(src)/Makefile looks better when we include
source files.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>