libtraceevent: Fix header installation

When we passed some location in DESTDIR, install_headers called
do_install with DESTDIR as part of the second argument.

But do_install is again using '$(DESTDIR_SQ)$2', so as a result the
headers were installed in a location $DESTDIR/$DESTDIR.

In my testing I passed DESTDIR=/home/sudip/test and the headers were
installed in: /home/sudip/test/home/sudip/test/usr/include/traceevent.

Lets remove DESTDIR from the second argument of do_install so that the
headers are installed in the correct location.

Signed-off-by: Sudipm Mukherjee <sudipm.mukherjee@gmail.com>
Reviewed-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Cc: Sudipm Mukherjee <sudipm.mukherjee@gmail.com>
Cc: linux-trace-devel@vger.kernel.org
Link: http://lore.kernel.org/lkml/20191114133719.309-1-sudipm.mukherjee@gmail.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>