Makefile: Exclude tests/Makefile in unconfigured tree

Makefile: Exclude tests/Makefile in unconfigured tree

Since commit dbfe06c62ccedc5b64e1c6466445133dd50f6de1 (build:
split unit test builds to a separate makefile fragment),
in absence of config-host.mak an undefined $(SRC_PATH) breaks
`make distclean' due to /tests/Makefile not being include'able.

Fix by only including when config-host.mak is present.

Signed-off-by: Andreas Färber <andreas.faerber@web.de>
Cc: Anthony Liguori <aliguori@us.ibm.com>
Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>