Private headers are listed in source[] while public headers are listed
in public[] for icuuc and icu18n targets.

This change is to let trybot analyze step recompile icuuc or icu18n even
when the only change in CL is in a header file (public or private).

BUG=632438
TEST=`gn check <out_dir>` does not complain.
TEST=`gn ref <out_dir> third_party/icu/source/common/umutex.h` matches icuuc.
TEST=`gn ref <out_dir> third_party/icu/source/common/unicode/unistr.h`
     matches icuuc.

R=brettw@chromium.org

Review URL: https://codereview.chromium.org/2186343002 .