ipmi plugin: fix sign compare warning

  CC       src/ipmi_la-ipmi.lo
src/ipmi.c: In function ‘sensor_unit_to_type’:
src/ipmi.c:360:21: warning: comparison of integer expressions of different signedness: ‘int’ and ‘long unsigned int’ [-Wsign-compare]
   for (int i = 0; i < STATIC_ARRAY_SIZE(ipmi_db_type_map); i++)
                     ^