Makefile: Exclude tests/Makefile in unconfigured tree

Since commit dbfe06c6 (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: NAndreas Färber <andreas.faerber@web.de>
Cc: Anthony Liguori <aliguori@us.ibm.com>
Signed-off-by: NStefan Hajnoczi <stefanha@linux.vnet.ibm.com>

Makefile

@@ -168,7 +168,9 @@ qemu-ga$(EXESUF): LIBS = $(LIBS_QGA)
 
 gen-out-type = $(subst .,-,$(suffix $@))
 
+ifneq ($(wildcard config-host.mak),)
 include $(SRC_PATH)/tests/Makefile
+endif
 
 $(qapi-dir)/qga-qapi-types.c $(qapi-dir)/qga-qapi-types.h :\
 $(SRC_PATH)/qapi-schema-guest.json $(SRC_PATH)/scripts/qapi-types.py