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>