kbuild: Use $(obj)/%.cc to fix host C++ module builds

Use $(obj)/ instead of $(src)/ prefix when building C++ modules for
host, as explained in commit b1992c37 ("kbuild: use $(src) instead
of $(srctree)/$(src) for source directory").  This fixes build failures
of 'xconfig':

    $ make O=build/ xconfig
    make[1]: Entering directory '/data/linux/kbuild-review/build'
      GEN     Makefile
    make[3]: *** No rule to make target '../scripts/kconfig/qconf-moc.cc', needed by 'scripts/kconfig/qconf-moc.o'.  Stop.

Fixes: b1992c37

 ("kbuild: use $(src) instead of $(srctree)/$(src) for source directory")
Reported-by: Rolf Eike Beer <eb@emlix.com>
Signed-off-by: Nicolas Schier <n.schier@avm.de>
Tested-by: Rolf Eike Beer <eb@emlix.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>