Fix building with GCC

Add a missing include for uintptr_t usage.

Change-Id: I389aa19bb04fc23154df5896237a8f3fa8488736
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/117072
Reviewed-by: Thomas Sepez <tsepez@google.com>
Commit-Queue: Lei Zhang <thestig@chromium.org>
Reviewed-by: Tom Sepez <tsepez@chromium.org>
diff --git a/core/fxcrt/fx_memory.cpp b/core/fxcrt/fx_memory.cpp
index 9c03916..8a22536 100644
--- a/core/fxcrt/fx_memory.cpp
+++ b/core/fxcrt/fx_memory.cpp
@@ -6,6 +6,7 @@
 
 #include "core/fxcrt/fx_memory.h"
 
+#include <stdint.h>  // For uintptr_t.
 #include <stdlib.h>  // For abort().
 
 #include <iterator>