qubes-rpc has its own Makefile that's responsible for building some
executables. The root Makefile was installing qubes-rpc files. To make
qubes-rpc a bit more indepdent from core-agent root Makefile and to ease
potential maintainer work on packaging qubes-rpc separately, the
installation has been moved to qubes-rpc Makefile. Moreover that should
make the Makefiles easier to read and maintain.
