Fix fx_string.h compile error under chromium checkouts.

This gets included in chromium's pdfium_engine.cc, and thus must pass a
higher error level.  There's probably a follow-up to check why the FPDF_ api
doesn't insulate chromium from this file.

TBR=thestig@chromium.org

Review URL: https://codereview.chromium.org/1127043004