fxcrt: fix building against system dragonbox in stream_write_utils

This was the only location that included dragonbox.h, and the '#if'
style is similar to
https://pdfium-review.googlesource.com/c/pdfium/+/154490.

Change-Id: I58580a9283c64dba135f19af9a9325c04831e3af
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/158950
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Commit-Queue: Tom Sepez <tsepez@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>

core/fxcrt/stream_write_utils.cpp

diff --git a/core/fxcrt/stream_write_utils.cpp b/core/fxcrt/stream_write_utils.cpp
index 927f64b..4c22649 100644
--- a/core/fxcrt/stream_write_utils.cpp
+++ b/core/fxcrt/stream_write_utils.cpp
@@ -10,7 +10,12 @@
 #include <ostream>
 
 #include "core/fxcrt/span.h"
+
+#if defined(USE_SYSTEM_DRAGONBOX)
+#include <dragonbox/dragonbox.h>
+#else
 #include "third_party/dragonbox/src/include/dragonbox/dragonbox.h"
+#endif
 
 namespace {