package/tzdate: use classic 'fat' format for uClibc/glibc compatibility

Fixes:
https://gitlab.com/buildroot.org/buildroot/-/jobs/1019385940

FAIL: test_run (tests.core.test_timezone.TestGlibcNonDefaultLimitedTimezone)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/builds/buildroot.org/buildroot/support/testing/tests/core/test_timezone.py", line 66, in test_run
    self.assertEqual(tz[0].strip(), "EST")
AssertionError: '' != 'EST'

Commit 7868289f (package/zic: bump version to 2020f) bumped the zic
version to 2020f, which changed the default output format from the classic
"fat" format to the new "slim" format:

https://github.com/eggert/tz/commit/6ba6f2117b95eab345a7ed9159cef939e30c4cd3



The slim format is unfortunately not supported by glibc < 2.28 or uClibc, so
explicitly request the classic "fat" format.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>