libtraceevent: Remove *.3 and *.m documentation files from build directory

The clean target in the make file for the documentation used just *.3 and
*.m to remove the temporary files. These files are built in the build
directory, and the clean does not affect them if a remote directory is
used to build the documentation.

Link: https://lore.kernel.org/linux-trace-devel/20211217003445.7d4c9025@oasis.local.home

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