unbreak "make" from vpath-built tests directory

Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>
Signed-off-by: NBlue Swirl <blauwirbel@gmail.com>

tests/Makefile

 -include ../config-host.mak
+-include $(SRC_PATH)/rules.mak
 
 $(call set-vpath, $(SRC_PATH)/tests)