Add missing slashes in paths for make uninstall

Add missing slashes in paths for make uninstall

Running make uninstall used to fail to remove binaries:
rm -vf /usr/local/binyosys /usr/local/binyosys-config #...etc

Fix Makefile so that it runs a command like this:
rm -vf /usr/local/bin/yosys /usr/local/bin/yosys-config #...etc