durations: fix an incorrect preprocessing condition
durations: fix an incorrect preprocessing condition

H2O_USE_LIBUV is always defined, despite DISABLE_LIBUV being set
on CMAKE invocation. See "include/h2o/socket.h" for proof.

yannick/h2o,cwyang/h2o,cwyang/h2o,devnexen/h2o,yannick/h2o,devnexen/h2o,cwyang/h2o,h2o/h2o,devnexen/h2o,devnexen/h2o,h2o/h2o,h2o/h2o,cwyang/h2o,yannick/h2o,yannick/h2o,h2o/h2o,devnexen/h2o,devnexen/h2o,devnexen/h2o,yannick/h2o,cwyang/h2o,yannick/h2o,yannick/h2o,h2o/h2o,yannick/h2o,cwyang/h2o,cwyang/h2o,h2o/h2o,h2o/h2o,cwyang/h2o,h2o/h2o,h2o/h2o,devnexen/h2o,devnexen/h2o,yannick/h2o