Move //base to public dep

The 'fuzzer_support' target uses base/ in public headers, therefore to
transitively inform dependent targets about the dependency, base/ must
go in 'public_deps'.

This change is required for the support of abseil optional, which is
transitively included from base/at_exit.h.

Bug: chromium:1202909
Change-Id: I804e3a689e2436201e3dcebefab4a28338d60522
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/deps/icu/+/2854155
Reviewed-by: Oliver Chang <ochang@chromium.org>