trace-cmd: Have etcdir always be at the root directory

There's a test if prefix == /usr/local and if so, then etcdir is set to
"/etc", otherwise it is set to $(prefix)/etc, which is not correct.

Always have etcdir = "/etc".

Link: https://lore.kernel.org/linux-trace-devel/20201216044213.152508008@goodmis.org

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