Add build_config.h to core/fxcrt/fx_extension.h

core/fxcrt/fx_extension.h checks USE_SYSTEM_ICUUC, which is
defined in build/build_config.h.

Change-Id: I2d48d61b62c422652104199259da37556fbce36c
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/106130
Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
diff --git a/core/fxcrt/fx_extension.h b/core/fxcrt/fx_extension.h
index ffdb7a2..ae94479 100644
--- a/core/fxcrt/fx_extension.h
+++ b/core/fxcrt/fx_extension.h
@@ -12,6 +12,8 @@
 #include <time.h>
 #include <wctype.h>
 
+#include "build/build_config.h"
+
 #if defined(USE_SYSTEM_ICUUC)
 #include <unicode/uchar.h>
 #else