libtracefs: Keep build_install file around

Instead of rebuilding the build_install files every time the uninstall is
created, make it depend on the prefix, as if the prefix is the same, then
the uninstall should be the same.

Link: https://lore.kernel.org/linux-trace-devel/20201216043922.025513420@goodmis.org

Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>