Makefile.target: Build gdbstub-xml.o only when
TARGET_XML_FILES is not empty.

Signed-off-by: Ákos Kovács <akoskovacs@gmx.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>