build-sys: fix make install regression

build-sys: fix make install regression

Since bd6092e Replace qmp-commands.hx by docs/qmp-commands.txt
make install fails if building out of source-tree.

Cc: Marc-André Lureau <marcandre.lureau@redhat.com>
Cc: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Riku Voipio <riku.voipio@linaro.org>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Message-id: 1474362094-2293-1-git-send-email-riku.voipio@linaro.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>