tests: Makefile: include dependency files

Otherwise 'make check' won't recompile files that need to be recompiled
because of header changes.

To reproduce the bug, run:

 $ make check  # succeeds
 $ echo B0RKED > hw/mc146818rtc_regs.h
 $ make check  # is supposed to try to rebuild tests/rtc-test.o and fail

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>

tests/Makefile

@@ -147,3 +147,5 @@ check-qtest: $(patsubst %,check-qtest-%, $(QTEST_TARGETS))
 check-unit: $(patsubst %,check-%, $(check-unit-y))
 check-block: $(patsubst %,check-%, $(check-block-y))
 check: check-unit check-qtest
+
+-include $(wildcard tests/*.d)