Include the appropriate header for BUILD_FUSCHIA
Include the appropriate header for BUILD_FUSCHIA

Without this, attempting to roll crashpad in Chromium gives this
presubmit warning:
third_party/crashpad/crashpad/util/file/file_io_posix.cc:69
  OS_FUCHSIA macro is used without including build/build_config.h.

R=f1b5a91d4d6ad523f2610114591c007e75d15084@chromium.org

Bug: none
Change-Id: Ie2d1df574773b66687948a481b9b31012427a3c3
Reviewed-on: https://chromium-review.googlesource.com/830258
Commit-Queue: Ilya Sherman <132f3a2061152e8061cfd42aff9316f15e66d3ce@chromium.org>
Reviewed-by: Mark Mentovai <f1b5a91d4d6ad523f2610114591c007e75d15084@chromium.org>

chromium/crashpad,chromium/crashpad,chromium/crashpad