hwclock: fix signed/unsigned comparison warning on alpha

hwclock: fix signed/unsigned comparison warning on alpha

Fixes the following warning:
sys-utils/hwclock.c: In function 'manipulate_epoch':
sys-utils/hwclock.c:1465:17: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   if (epoch_opt == -1)

Full build log available on:
https://buildd.debian.org/status/fetch.php?pkg=util-linux&arch=alpha&ver=2.26.2-9&stamp=1440078034

Detected by/via:
https://qa.debian.org/bls/packages/u/util-linux.html

Please note that this has never actually been (build-)tested on alpha,
but should hopefully resolve the warning. Note also that limits.h is
already included.

Signed-off-by: Andreas Henriksson <andreas@fatal.se>