grn_time_t_from_tm: add a comment about error check
grn_time_t_from_tm: add a comment about error check

It describes why we don't use mktime() return value for error check.

groonga/groonga,komainu8/groonga,groonga/groonga,groonga/groonga,komainu8/groonga,komainu8/groonga,kenhys/groonga,komainu8/groonga,kenhys/groonga,groonga/groonga,komainu8/groonga,kenhys/groonga,kenhys/groonga,komainu8/groonga,kenhys/groonga,kenhys/groonga,kenhys/groonga,komainu8/groonga,groonga/groonga,kenhys/groonga,groonga/groonga,groonga/groonga,groonga/groonga,komainu8/groonga