debian/rules: Regression fix for usage of DEB_HOST_ARCH vs. DEB_BUILD_ARCH....

debian/rules: Regression fix for usage of DEB_HOST_ARCH vs. DEB_BUILD_ARCH. Switching back to DEB_HOST_ARCH to not break cross-builds. Thanks to Guillem Jover for teaching me about the difference between those two options. (Closes: #822842).