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>
