Add casts to work around harmless -Werror warnings from clang 10.0.0,
such as:

usr.sbin/timed/timed/networkdelta.c:160:13: error: implicit conversion from 'long' to 'float' changes value from 9223372036854775807 to 9223372036854775808

[-Werror,-Wimplicit-int-float-conversion]
  float ap = LONG_MAX;            /* bounds on the median */
        ~~   ^~~~~~~~

Direct commit to stable/{10,11,12}, since timed has been removed from
FreeBSD 13.