libtraceevent: Fix dependencies in Documentation Makefile

Modify the Documentation Makefile to only rebuild the man pages when their
source has been modified.

Link: https://lore.kernel.org/linux-trace-devel/20210408214200.2063946-3-rostedt@goodmis.org

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