gzip reference dumps while copying to ref-dump-dir.

Remove the additional step of gzipping reference dumps manually.

Test: utils/create_reference_dumps.py --version current -ref-dump-dir
      $ANDROID_BUILD_TOP/prebuilts/abi-dumps/vndk; mm -j64 in
      external/libexif.

Change-Id: Ib22a3b4fa90e4f849529efc4b03ab43d87996458