src/pmdas/lmsensors/GNUmakefile: fix build for "I don't have sensors" case
src/pmdas/lmsensors/GNUmakefile: fix build for "I don't have sensors" case

Was failing, if lm_sensors or sensors packages not installed, e.g. on
NetBSD.

adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp