test-qga: add missing qemu-ga tool dependency

this fixes running 'make check-unit' without running 'make all' beforehand:

$ make check-unit
  ...
  GTESTER tests/test-qga
**
ERROR:tests/test-qga.c:73:fixture_setup: assertion failed (error == NULL): Failed to execute child process "/build/qemu/qemu-ga" (No such file or directory) (g-exec-error-quark, 8)
make: *** [check-tests/test-qga] Error 1

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20170911210129.5874-1-f4bug@amsat.org>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

tests/Makefile.include

@@ -832,7 +832,8 @@ endif
 qtest-obj-y = tests/libqtest.o $(test-util-obj-y)
 $(check-qtest-y): $(qtest-obj-y)
 
-tests/test-qga: tests/test-qga.o $(qtest-obj-y)
+tests/test-qga$(EXESUF): qemu-ga$(EXESUF)
+tests/test-qga$(EXESUF): tests/test-qga.o $(qtest-obj-y)
 
 SPEED = quick
 GTESTER_OPTIONS = -k $(if $(V),--verbose,-q)