lttng-modules: do not search in non-existing folder during install

When CONFIG_TRACEPOINTS is not enabled in kernel config - module
compilation is skipped, which causes the ${D}/${nonarch_base_libdir} not
to be created.

This fails later in do_install:append() due to the fact that find
command in executed for non-existing folder.

Check for folder existence before find command in executed.

Signed-off-by: Andrey Zhizhikin <andrey.z@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>