Makefile.target: Build gdbstub-xml.o only when
TARGET_XML_FILES is not empty.
Signed-off-by: NÁkos Kovács <akoskovacs@gmx.com>
Reviewed-by: NPeter Maydell <peter.maydell@linaro.org>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>