Fix Mac OS X buildbot

packet-s7comm.c:962: warning: declaration of 'time' shadows a global declaration

Change-Id: I4020b780e787a91c42f08f7b7b1c7feabcabdc62
Reviewed-on: https://code.wireshark.org/review/3987
Reviewed-by: Alexis La Goutte <alexis.lagoutte@gmail.com>