hunspell: Update the include path of base/string_piece.h to its new location.

string_piece.h was moved into base/strings/ in r191206 -
https://chromiumcodereview.appspot.com/12982018/

TBR=brettw@chromium.org

Review URL: https://chromiumcodereview.appspot.com/14426011

git-svn-id: http://src.chromium.org/svn/trunk/deps/third_party/hunspell@195968 4ff67af0-8c30-449e-8e8b-ad334ec8d88c
1 file changed