Limit the scope of a using statement in CFX_SystemHandler's header.

Change-Id: I82ab33def7ef35e72de2a5bcc1a5011d31dd3640
Reviewed-on: https://pdfium-review.googlesource.com/c/48070
Commit-Queue: Lei Zhang <thestig@chromium.org>
Reviewed-by: Tom Sepez <tsepez@chromium.org>
diff --git a/fpdfsdk/cfx_systemhandler.h b/fpdfsdk/cfx_systemhandler.h
index af55187..7c555b6 100644
--- a/fpdfsdk/cfx_systemhandler.h
+++ b/fpdfsdk/cfx_systemhandler.h
@@ -12,8 +12,6 @@
 #include "core/fxcrt/fx_system.h"
 #include "core/fxcrt/unowned_ptr.h"
 
-using TimerCallback = void (*)(int32_t idEvent);
-
 class CFFL_FormFiller;
 class CPDF_Document;
 class CPDF_Font;
@@ -23,6 +21,7 @@
 class CFX_SystemHandler {
  public:
   static constexpr int32_t kInvalidTimerID = 0;
+  using TimerCallback = void (*)(int32_t idEvent);
 
   explicit CFX_SystemHandler(CPDFSDK_FormFillEnvironment* pFormFillEnv);
   ~CFX_SystemHandler();