conf/: fix parallel install

Do not reference the upsmon.conf.sample twice, otherwise install, with
a high number of make jobs, may fail, like so:
    http://autobuild.buildroot.net/results/256/2567e13cd5bc702bc3a38a1d6fc8e34022cc7db5/build-end.log

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>

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This is not a rare occurence, as my testing managed to trigger the
issue in about 1 test out of 10 on average, on a not-so-fast machine.