build-sys: fix make install regression

Since bd6092e4 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: NRiku Voipio <riku.voipio@linaro.org>
Reviewed-by: NMarc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: NMarkus Armbruster <armbru@redhat.com>
Message-id: 1474362094-2293-1-git-send-email-riku.voipio@linaro.org
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>

Makefile

@@ -431,7 +431,7 @@ endif
 install-doc: $(DOCS)
 	$(INSTALL_DIR) "$(DESTDIR)$(qemu_docdir)"
 	$(INSTALL_DATA) qemu-doc.html  qemu-tech.html "$(DESTDIR)$(qemu_docdir)"
-	$(INSTALL_DATA) docs/qmp-commands.txt "$(DESTDIR)$(qemu_docdir)"
+	$(INSTALL_DATA) $(SRC_PATH)/docs/qmp-commands.txt "$(DESTDIR)$(qemu_docdir)"
 ifdef CONFIG_POSIX
 	$(INSTALL_DIR) "$(DESTDIR)$(mandir)/man1"
 	$(INSTALL_DATA) qemu.1 "$(DESTDIR)$(mandir)/man1"