platform2: Specify default value for USE_wimax in common.gypi

BUG=chromium:313512
TEST=`FEATURE=test emerge-link platform2`
TEST=`FEATURE=test emerge-lumpy platform2`
TEST=`FEATURE=test emerge-daisy platform2`

Change-Id: I5daa0360bd9d92c9c113e0feb5eb4dc203dfa90a
Reviewed-on: https://chromium-review.googlesource.com/175208
Reviewed-by: Mike Frysinger <vapier@chromium.org>
Tested-by: Ben Chan <benchan@chromium.org>
Commit-Queue: Mike Frysinger <vapier@chromium.org>