Move fx_safe_types.h to include/ directory.

Small bit of OCD here, since this file is included cross-library
(i.e. from fpdfsk), it can't be in src/.  In other words, the
following should be empty:
    grep -R 'include.*core/src/' fpdfsdk

Fix some IWYU in it at the same time.

R=thestig@chromium.org

Review URL: https://codereview.chromium.org/1147353006