Pack libfec.so into otatools.zip.

delta_generator now has a dependency on libfec.so [1], for generating
A/B incrementals.

[1] https://android-review.googlesource.com/c/platform/system/update_engine/+/854154

Bug: 122280472
Test: `m -j otatools-package` with aosp_marlin-userdebug. Check that
      `otatools.zip` contains libfec.so.
Change-Id: Ia6642bb2c7ccb0f89ad3079bdf65e4dbffee13f5